Copypasta from The Citadel:

Q: What is the bool __cdecl X2mFile::InstallCostumePartSets(void): Overflow, cannot assign suitable(s) partset(s) id(s) error and what to do about it?

A: You've got too many costume, accessory or transformation mods installed on one or more CaC races. Try uninstalling some mods in those categories.
Explanation: Each race can have a maximum of 1000 Partsets (0-999). This is due to the way the game reads what we call the MODEL element in bcs, which can only have 3 digit decimal numbers.
Now these 1000 Partsets are purely theoretically, as the first 255 Partsets are reserved for character creation, vanilla uses some of the others already and a few are special use-cases.
The actual numbers, as of 1.13, were:
Vanilla FRI.bcs has 380 free Partsets that the installer can use, HUF has 369, HUM 379, MAF 371, MAM 381 and NMC 381.
Now this does not mean you can install 369 mods on HUF, for example. A 3 stage transformation mod needs 3 Partsets, if you have, let's say, a mod that comes with 4 outfits (bust, wrist, pants, boots) it needs at least 4, but depending on how the mod is set up, it can have as many as 16, and accessory mods need 1 Partset per included accessory.
Also note that with transformations, the needed Partsets need to be continuous (291, 292, 293 for example, not 256, 292, 293), so having three free Partsets might not be enough to install a three stage transformation skill.