1) Few or plenty?
Do you prefer having a small bunch of races available to PCs, such as the PHB set or a cherry-picked group of favourites? Or do you think a large array available is better? Or do you even think the sky's the limit and would like to have as many as you can find for the edition you're currently playing?
2) Common or variable?
Do you prefer a more monolithic approach on racial mechanics i.e. all PCs of the same race X or subrace Y must use the same stats, or do you like having mechanical variants for each race and/or subrace (without narrating them as a different group)?
3) Native or outsiders?
Do you want playable races strictly from the fantasy setting you're adventuring in, or do you consent to PCs of a race that normally doesn't belong to it? This is a more nuanced question... you can make a distinction between the case of a race that belongs to the setting but not the world (like are you ok with PC races from the elemental or the outer planes) and the case of a race that belongs to a different setting entirely (like a Forgotten Realms race in Dark Sun) or even a different genre altogether (like a Star Wars or DCU race).
Bonus question (kind of a combination of questions 2 and 3): how do you feel about using different versions of a race from different settings? Examples could be allowing Zendikar elves or Ravnica goblins (assuming you already have native elves or goblins), would you be ok with allowing the mechanical variant (i.e. different stats and abilities) from another setting? If you would allow the mechanic, would you disallow, allow or even require the narrative that the race comes from another world?
writing this from the perspective of a [theoretical] player
1) I don't mind lots of races to pick from
but i don't want them all happening at once, and i don't like having redundant races in a general sense, pick the ones you're going to have in a setting and stick to that, don't start going all kitchen sink on things,
Personally i'd sort races into three groups:
Common: humans, elves, dwarves, halflings, orcs, those who are standardly present in all settings unless specifically noted otherwise, around 60-80% of the general population, may be treated as Uncommon where Exclusive races are the Common ones. Found in every setting
Uncommon: dragonborn, tieflings, gnomes and suchlike, present in
most settings but to not to the degree of the common races, could be treated as a common race in an apropriately themed setting, 37-17% of the general population. found in any but not every setting
Exclusive: I don't really know what the non-standard races are myself but, These are races that are treated as exclusively odd to see in a setting outside of the ones they're not 'native' to but considered as a Common race inside those settings, if one appears in a setting they are not native to they should be treated as a foreigner to that land, 3% of the general population.
2) I lean hard to the side of common myself, fixed ASI and abilities, yes have subclasses that provide more granular choices within a race is fine, but fixed choices, just because you wanted to become a bard your inherent dwarven hardiness can't be 'trained' into a CHA bonus, that's what the high number from your rolled scores/point buy/standard array is for.
3) I essentially answer this in 1 already didn't I, I guess, i'd allow them but with the caveat that their race is specifically noted as strangers to the setting, sure maybe they might have been born and raised in the setting but the fact that they're there is treated as an anomaly in a general sense.
Bonus) So long as you can justify it narratively it's fine IMO, developed in some highly specific environment, exposed to some magic that changed them, travellers from another dimension, whatever, great, just give them a reason to be there, but if they're functionally an entirely different race in all but name [like comparing DnD elves with say, Santa's elves] reskin them or just rename them i'd say.
