Equinox Road has rules for turning Changelings into True Fae. The fact is that, by the book, supernaturals are generally STAGGERINGLY overpowered compared to mortals so if you want them to face a True Fae and "win" your best bet is probably just to keep combat out of the equation. Faeries are all about rules, after all.
Neither and both.
Unlike in Previous editions, CR isn't used to directly create combat encounters, instead the XP values of the creatures ( which is tied directly to CR) is used to determine how many creatures can be used in the encounter. Instead, CR tells you the upper maximum difficulty of the monster, assuming a party of 4. One of the reasons why they moved away from using Level instead of CR, is so that you did not confuse the CR of the Monster with the level of the characters fighting the monster.
So, basically a CR1 monser means that if your characters are level 1, this will be a challenge for them. If they are level 2, it will be pretty easy depending on how you use the xp budget to put 1 or more monsters into the battle. If the monster is CR 1/4, then that means that if you have a single character alone, without the party it will be a challenge for them, 1/8 means 2 will be a challenge for an indvidual, and a CR 0 means it's not a challenge for anybody. You will notice that CR 1/8th has an XP value of 25 while a CR 0 has an xp value of 10.
But the important criteria you want to use when buliding an encounter is your XP budget. Use CR to only filter out which creatures are too deadly for the level of your party of any size.
Challenge Rating, page 56
Much of the advice in this section focuses on the XP values of
monsters and encounters, as opposed to their challenge rating.
Challenge rating is only a guidepost that indicates at what level
that monster becomes an appropriate challenge. When putting together
an encounter or adventure, especially at lower levels, exercise
caution when using monsters whose challenge rating is higher than the
party’s level. Such a creature might deal enough damage with a single
action to overwhelm PCs of a lower level. Even though an ogre has a
challenge rating of 2, for example, it can kill a 1st-level wizard or
sorcerer outright with a single blow.
When building encounters, use XP values. Those XP values along with the relevant multipliers for party size and number of monsters, are what are important when determining how deadly a monster is to the party or to a character. CR is only a filter for the "upper limit" of which monsters you should choose from when using the XP budgets.
So in the end, trying to understand how many party members a CR is coordinated against, becomes a bit of a red herring for when it matters. The answer is "4", but that isn't really the answer you want, what you want to focus on is your XP budget, and the multipliers for party size and number of monsters.
Best Answer
[This all assumes you're looking for a medium encounter, as defined in the DMG. Adjust upward or downward for easy, hard, or deadly, as instructed in the various sections.]
By the DMG
"Creating a Combat Encounter" (pp.81-84) has your instructions. For example, a "medium" encounter for a party of 5 level 3 PCs would have an adjusted XP budget of 750--1125.
A CR 1/4 creature is valued at 50XP (MM. p.9), so we can fiddle with numbers and encounter multipliers (DMG p.82) to see that 6 creatures would be too few, 9 would verge on "hard." So your answer's 7 or 8, possibly 9.
By XGtE
"Encounter Building" (pp.88-91) has more guidelines. There we'd see that for level 3 characters I should provide CR1/4 enemies in a 2:1 ratio. So XGtE recommends 10 of them.
By the sheep
I say throw anywhere between six and ten at them, depending on what makes sense in the fiction. Then use your gameplay and game-time decisions to adjust difficulty as it goes. If ten's too many have a few cowardly ones break off the first time one goes down. If six is too few, have them fight smart. (Placing the encounter in terrain/location that you can choose to use to your advantage--or not--is key to this, I believe. Personally I'm not a fan of bringing in reinforcements unless the party knows they're in a monster-ridden location like a hostile camp, but others report success with that tactic.)
The quick calculation I use is to try and make sure the monsters are dishing out damage about 1/2 the rate the party generally does.* Your L3 adventurers are probably dishing out about 10 or 12 dmg per round, hitting 2/3 of the time, for a party total of 35-40. How many CR 1/4 does it take to dish out 1/3 of that? Say they do 6 dmg per round at the same hit-rate, we're looking at 5 monsters. But it's nicer to have too many than to have too few--it's much easier to "dial back" as a GM--so that's where the recommendation of 6-10 comes in.
* - This is based on the notion of six encounters and two short rests per adventuring day; if a "medium" encounter starts its lifecycyle dishing out 1/2 the party's damage and decreases somewhat-linearly (as monsters start dropping) to 0, the time-averaged damage output sits around 1/4 the party's. But monsters don't always drop that quickly (though these little ones will), so the real time-average is a little higher, call it 1/3. Which is what I really want from a medium encounter so that two of them will make the party want for a short rest, and an "easy" should be easier than that and a "hard" has room to be noticeably harder.