1. Playing on high caps and having a lot of units reinforced to an ai planet at once would possibly get you 1200.
2. Also, being at an ai planet with enough military that they are not in cold storage, and then leaving while a ton of them are still alive and thus all go into cold storage a once.
3. Also, loading older savegames from before the barracks mechanic.
4. To get 201 or more units, all it takes is a larger-than-average reinforcement, high caps, multiple reinforcements at once to the same planet, or again #2 from above.