I've taken half an hour just looking through the auction house, and I have taken my own experience and various forum posts into account, and these are my findings:

The maximum number of sockets per equipment type varies. It seems to be independent of item rarity (magic and rare items, legendaries are something special). Instead, it seems to depend entirely on the body type of the item.

These are the maximums I was able to identify:

First of all: Shoulders, belts, gloves, and boots can't have sockets at all (more on that later).

Helmets, rings, amulets, weapons, and offhands:

• one socket

Pants

• two sockets

Chest Armor

• three sockets

Legendary items are a special case: They always have a certain fixed amount of sockets per named item, but just for completeness: The maximum number of sockets on legendaries I could find was two.

There's something I have to add about Shoulders, belts, gloves, and boots: Even though there seem to be no belts with sockets, it is theoretically still possible that belts can have sockets, and simply nobody has found one yet. I come to this conclusion from a simple fact: Shoulders, gloves, and boots have no search option for "has sockets" in the auction house, indicating that it is indeed not possible for them to have one. Belts, however, do have this search option. It is entirely possible that Blizzard just left it in by accident, though.

Another interesting fact about items and sockets: Items have a fixed amount of random properties (this can differ, even for same rarity and body placement). When an item is dropped, or identified, the contents of these properties are randomly rolled out. Sockets, no matter how many there are in the item slot, take up exactly one of these random properties.

This make for an interesting conclusion: If you're unlucky, sockets may take away important stats from your item. In general, pure item stats will always be higher than you could at to it with gems. For example, perfect strength gems provide +22 strength. In comparison, items of roughly the same level seem to hit an upper cap of around +60 strength. This means that if you, for example, identify a rare chest armor piece and it finds +50 vitality, +20 experience per kill and +1.2% chance to stun on hit, plus one socket, then that last socket could have been a slot for +50 strength, whereas now you can only try to compensate for it with gems.

On the contrary, however, this becomes a nice bonus when you actually want to stack a certain stats. Items can't roll the same bonus stat twice. If we go with the example from above, you now can circumvent this fact by adding another +22 vitality gem into the socket, thus giving you even more life on the item (good for tanks, for example).

This thread discusses all of this (notice Bashiok's post later in the thread, acknowledging these facts).

Kormac the Templar

Strengths

• Good support character for higher difficulties, ranged classes, or while initially learning the game. Can heal and taunt from the player when they are at low health.
• Decent Tank, as he can be equipped with one handed weapons and a shield in addition to rings and amulet.
• Can provide good DPS through the use of items that give "Thorns" type damage on hit, as he will charge into the thick of combat and be hit by packs of enemies.
• Level 20 "Inspire" ability helps with resource regeneration for the player, leading to more opportunities for ability use

Weaknesses

• Susceptible to being surrounded and killed fairly easily if not geared properly. His charge in and hack style gets him killed a lot by high difficulty elites.
• Not a particularly good direct damage dealer

Suggested follower for: Demon Hunter, Wizard, Witch Doctor, though he is a very useful follower for any class at more difficult stages.

Suggested Gear Stats: Vitality, Strength, "Thorns" type damage on being struck in melee

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PLEASE NOTE: Follower stats are increased by a 2.5x multiplier on the 4 base stats, as denoted in this post. Therefore, it is worthwhile to give followers items that increase these stats primarily.

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Lyndon the Scoundrel

Strengths

• Good ranged support character for melee classes, has decent crowd control ability
• Offensively focused, can deal more direct damage than the Templar and Enchantress
• Good survival ability, high dexterity boosts his dodge chance and helps him avoid taking damage. Level 15 "Vanish" ability allows him to drop aggro and heal himself.
• Can provide a boost to critical hits using either of his Level 25 abilites "Anatomy" or "Hysteria" depending on preference
• Witty and humorous, making his dialogue more interesting than other followers

Weaknesses

• Not extremely durable if he gets cornered
• Crowd control abilities are inferior to those of the classes he is ideally supporting
• Like the Enchantress, is prone to pulling groups before a player is ready

Suggested follower for: Barbarian at early difficulties where Templar healing is not necessary, Monk at all difficulties.

Suggested Gear Stats: Dexterity, Vitality, +Damage, +AtkSpd, Crit%

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Eirena the Enchantress

Strengths

• Good ranged support character for melee classes, has excellent crowd control ability
• Good balance of support spells and damage dealing
• Can buff the attack speed of the hero
• Her Charm ability even works on elites (non-boss), which will completely incapacitate them for a short time or turn them against their allies. Very helpful on high difficulties.
• High intelligence allows good mitigation of ranged attacks

Weaknesses

• Not durable, but can generally get out of trouble with excellent crowd control ability
• Damage is inferior to Scoundrel, and can also be inferior to a properly geared Templar
• Like the Scoundrel, will pull sometimes before player is ready

Suggested follower for: Barbarian at early difficulties where Templar healing is not necessary, Monk at all difficulties, Witch Doctor at any difficulty.

Suggested Gear Stats: Intelligence, Vitality, +Damage, +AtkSpd

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Basically you should pick the follower to support your playstyle. I personally prefer to have the Templar following me just about everywhere as I can count on him for an emergency heal and taunt when I get in trouble, he also provides increased resource regeneration so I can use my skills more often. However, there are obvious benefits at lower difficulties to use the Scoundrel, where a tank type may not be as necessary and the direct DPS increase helps get through mobs quicker. The Enchantress can also be a good choice due to her CC ability in late difficulties where you don't want to get overwhelmed by numbers.

THIS GUIDE IS CURRENT AS OF PATCH 2.0/REAPER OF SOULS