When They're Worth the Cost
For characters like animal instinct barbarians or monks with better unarmed attacks, there may not be much use in taking these abilities. But for various characters like the goblin wizard or even just a regular fighter looking for a guaranteed backup option, the benefits may outweigh the typically low costs.
Benefits of Ancestral Unarmed Attack
You've mentioned quite a few reasons it might be worthwhile to get an unarmed attack beyond the standard 'fist', including as a backup if weapons are taken away and freeing up whatever hands a weapon would use.
Having an additional free hand opens up quite a few Athletics actions like Trip and Grapple that render foes flat-footed to your entire party, as well as allowing the Grab an Edge reaction without needing to let go of a precious weapon if someone knocks you off a cliff.
Additionally as ESCE mentioned below, there's value in having multiple damage types available in case of enemies with resistances, immunities, weaknesses, or other circumstances. Dealing 1d6+10 on a bite against piercing weakness would potentially be better than 2d8 slashing with a striking weapon, even factoring the +1 accuracy.
And while a spiked gauntlet would be pretty good as an alternative for one of these cases, it deals less damage than many of these ancestral unarmed attacks and isn't finesse for characters that lack Strength.
Additionally, not all ancestral unarmed attacks deal physical damage—there's also options like an automaton's energy beam dealing fire.
Character-Specific Benefits
Virtually all characters are at least trained in unarmed attacks, while many are not trained in martial weapons like shortswords or warhammers. A wizard with a surprising d6 bite attack might come in handy compared to drawing out a dagger (requiring an open hand) and dealing less damage. Or an alchemist firing a laser out of their eye might make a good ranged option when there aren't any bombs left.
Not to mention that with some options there are more ancestry feats associated with the unarmed attack. Like the goblin's bite in the reaction Ankle Bite and Hungry Goblin, allowing more specialization and benefits using these unarmed attacks as you level up. Whether any ancestral unarmed attack has similar synergistic options will depend on the specific case, a goblin building around that bite attack by taking those ancestry feats seems like an interesting character with advantages over other attack options.
And with versatile heritages like dhampirs, beastkin, and tieflings, these options are available to virtually any character regardless of ancestry, with varying levels of additional support through higher level ancestry feats.
Flavor
There's also the simple benefit to being something like a sabre-tooth catfolk or an automaton shooting lasers out of their eye. Having an ancestral unarmed attack might be a core piece of character identity that a player will try to optimize around, rather than considering it as part of optimization.
Costs of Ancestral Unarmed Attack
In most cases, a 1st-level ancestry feat or heritage. Some ancestries like humans have great 1st-level ancestry feats (Natural Ambition comes to mind), but others have more niche uses/fewer options that appeal to every sort of character.
For one of your examples, a Catfolk adventurer may not be interested in being a bit luckier on Reflex saves, dancing, knowing the ancestral lore of their people, or fighting with their ancestral weapons. In this case, the opportunity cost of one 1st-level ancestry feat would be low, and the benefits are more likely to outweigh it.
And at the low levels these abilities are available, upgrading the unarmed attack isn't required. Retraining out of these feats when they become less useful relative to your magical weapon(s) is an option as that becomes a problem.
There's also the 3rd-level general feat Ancestral Paragon that allows characters to gain an additional 1st-level ancestry feat, granting even more opportunities to take these abilities in situations where options are limited.
Are They Worth It?
Probably not for many characters, particularly if an alternate option is more attractive, they have stronger attacks from a class, or they don't worry about the problems an ancestral unarmed attack would cover (knowing your GM isn't 'sadistic' certainly helps).
That said, characters with weaker alternative options from their ancestry/heritage/class, or who are looking for a particular attack like a laser eye could find these options good enough to take. And characters specializing in these attacks with synergistic, higher-level ancestry feats might be worthwhile on their own merit.
Best Answer
7 talisman or spellhearts, 11 with feats.
The rules for talismans state that:
Spellhearts behave the same way:
I can have an indefinite amount of talismans attached to various equipment, but for the purpose of this question I'll only count talismans that I can activate at any time, that is, talismans affixed to items that I am "wielding or wearing".
The rules for carrying and using items have this say about wielding and wearing items:
Unreasonable Reading
Say I have a dagger that's sheathed on my belt, meaning it's being worn, by the loosest reading of talisman rules that's sufficient to activate a talisman attached to the dagger, because I'm "wielding or wearing" the dagger. Likewise, if I hang a padded armor off my belt I'm also wearing it, and that's sufficient to activate a talisman affixed to it. By this reading, I can have an indefinite amount talismans that I can activate at any time: I could wear a lot of light weapons (talismans affixed to weapons typically trigger off some part of a strike so they wouldn't be practical, but I could activate affixed spellhearts without wielding these weapons). However, it's self-evident that this reading must be wrong, and that this bit about ambiguous rules applies:
So when the rules for talismans says that I "must be wielding or wearing an item to activate a talisman attached to it" there's an implied "as appropriate for the item in question". In other words, I must wield weapons or shields and must properly wear armor (not just hang it from a belt or stuff it in a bandolier).
Items
There's a few odd items to address, before we count how many talismans I could activate at any time.
Free-hand weapons such a gauntlets. The trait states that:
So while I can affix a talisman to my gauntlet, I can't activate the talisman while also wielding something in that same hand.
Attached weapons such a shield bosses and reinforced stocks. The trait states that:
The item and attached weapon can each be affixed with a talisman, because there's no indication that the attached-weapon+item pair count as one item, if anything the bit about salvaging the attached weapon from a destroyed item suggests otherwise. Although, not stated explicitly, an attached weapons is also wielded (or else it would be impossible to strike with it), so if I'm wielding the item, I can activate the talisman affixed to it as well as the one affixed to the attached weapon.
Both the handwraps of mighty blows and the bracers of armor allow affixing talismans, but both items have the invested trait, which states:
Given that affixing talismans is not a normal benefit of handwraps or bracers, I must invest the handwraps of mighty blows and the bracers of armor for their ability to be affixed with talismans to work.
Moreover, these items can be worn with any other item that does not have the usage "worn gloves/bracers", including armor.
So in total I can properly wear armor, properly wear and invest handwraps of mighty blows, properly wear and invest bracers of armor, and wield two weapons/shields each with an attached weapon, for a total of 7 items and therefore 7 talismans or spellhearts that I can activate at any time.
Feats
I have identified a few feats that can further increase the maximum.
Talismanic sage and Grand Talisman Esoterica each allow for one additional talisman or spellheart.
Juggle states:
So with this feat I could wield 2 light weapons with one hand. Unfortunately, the lightest combination of weapon and attached weapon totals 2L (a light firearm or a crossbow with either a bayonette or a reinforced stock), which is too heavy to juggle. That said, the Juggler archetype has two feats that further increase the maximum number of juggled items, so instead of wielding a weapon/shield with an attached weapon, I could juggle and wield 4 light weapons, thereby increasing the maximum number of talismans or spellhearts by 2.