hmm, well you cant set the count afaik and i dont think you can make the ai build nukes reliably either.
one way i can think of, although admitedly it is a little riciculous, is this: create a nuke building mode.
C:
whatever lets you to build nuke
current player commands exactly 0 nukes
A:
create 1 civ for CP at somewhere near a beacon
C:
CP brings at least 1 civ to beacon
A:
remove 1 civ for CP at beacon
give all men for current player at anywhere to player 9 (any player is fine, but either have a different one for each player or dont let everyone use it at once)
create 1 supply depot for CP at someplace out of the way (not crucial, if space is an issue have some stacked preplaced ones for player 12 or something and give 1 to CP)
give 1 nuclear silo owned by p12 at wherever it is to CP (have it preplaced, otherwise it'd need to be attached to a cc)
set nuke mode
C:
nuke mode is set
CP commands at least 1 nuke
A:
clear nuke mode
give all men for player 9 to CP
C:
current player commands exactly 0 nukes
A:
remove all supply depot for CP at someplace out of the way (or give it back to p12, whichever you're using)
that will let you make a nuke without it affecting your population count (except that it still counts to the 200 max). as long as people do it quickly, it shouldnt matter that they dont have their guys for a second.
None.