As the title says.
The weirdest race I've ever considered including in a campaign setting had only 5-10 personalities. Tops. For the entire race.
Or, rather, they were a crafted, barebones race, who had a set of reprogrammed personalities for people too lazy to code their own. When the civilization that made them fell, the secrets of how to code new personalities went bye-bye, and they were stuck with the factory defaults. Unlike a natural race, their personalities don't shift over time; a personality is static, until a particular stimulus flips a switch transferring them over to a different, more appropriate personality.
What their default personality was was purely "genetic"; there was no real genetic deviance in the race (a few physical templates, but those are (usually) inherited wholesale), so you could track genetic/personality "flow" over a large number of generations.
There were a few organizations that recaptured some of the "compiler", if you would, and used it to try to "crack" their personalities; they were moderately successful, but none of the new personalities would stick. However, they did discover several processes that allowed them to split up personalities to "breed" new ones. The new personalities were still entirely dependent on their parents, and would breed as if they were one of their parents' personalities, but it allowed them to make their own diplomats.
I thought they were cool; physically, they looked like albino humans (easier to dye their skin, hair, and eyes whatever color you wanted) with the crudest of physical features and a total lack of gender dimorphism. Their features would vary based on which personality they had at the moment, with some "codes" allowing further editing.
They would've been NPCs for the most part, a relic of a bygone era. Of course, if an old book of codes falls into the player's hands, well... Those relics might have gotten a rehaul.
