Usually, there is no problem with the party being a ragtag bunch of characters from distant lands. So long as each party member has some reason to be there and to stay there, the statistical unlikeliness of the scenario doesn’t really matter. This even let’s the group gain renown for being so unusual in its cast. But in the extremely rare case where the party makeup would be so infeasible (or literally impossible), I have done the following:
Determine a cap on the number of exotic characters in a party.
Tell my players about this beforehand and explain to them why there is a cap (they may find a flaw in your logic that means the problem never existed in the first place).
Note: I do this (and almost all character building) as part of a Session Zero. I will give details on the world beforehand so people can have some ideas for a character, but I ask that not too much of it is concrete. Then everybody build parts of the world and its characters together (this means people won’t walk in with a full character concept that simply cannot exist in the universe I am using).
Then ask the players if they have a specific way they want to resolve the issue (maybe they want random chance, or a priority list, or a friendly debate).
Ask if any of them would be just as happy picking a different race or a different character entirely. This can remove people from the pool immediately.
Ask if any of them want the race solely for mechanical benefits (I value character concepts and backstories above optimization).
See if this would be the first time playing an exotic race for any of them (this lets you remove everybody else from the pool).
If too many people still remain, I like to have them talk it through amongst themselves. I’ve never done anything else in this situation, and if you truly do need something quick and efficient it may be best to just roll a die or have them rate how much they want to play their character.