Suppose enemy X has a max driller count of 500. X's Queen provides 150 and 7 generators account for the other 350. X has a generator without any drillers guardingit. I take this generator, thereby reducing X's max driller count to 450.

Question: What happens to the 50 drillers X owns over X's max driller count?

He will keep the 50 drillers, but his factories will stop producing drillers until his max driller count falls below 450.

gis4gamer is correct.

It's a good tactical strategy to "lock" your enemies with excess drillers as they may stop producing drillers on the front-line of battle and be "stuck" with many drillers in the rear of their empire causing them the need to send them on long trips.