Evil Genius 2: World Domination

Simply add enough lockers (build mode, Barracks [bed icon] until you have 300 TOTAL capacity. Each locker adds plus 3 to capacity. You can get the Compact Lockers in the research menu later which gives you +20. Once you have 300 total capacity, wait or hire via Minion Manager until you have 300 minions in the lair. Achievement will pop then.