first thing real p90 is made by FN(Belgium, Fabrique Nationale) it has 50 rounds of 5,7 ammo...
however in Fallout universe we have non-existing HK(German, Heckler & Koch) version, then it is shooting 10mm ammo and having 24 rounds in a clip

why only 24 rounds... for example-because in 50's (fallout retro style) smg clips were small, there were no "3-column" or "4-column" clips, these were developed I believe in 80's or 90's.
this beauty for example has 50 round "4-column" clip
http://en.wikipedia.org/wiki/Spectre_M4and believe i didnt need wiki to post this thing about p90