I'm currently using an HP server power supply and Parallel Miner breakout board: https://www.parallelminer.com/product/baikal-bk-n70-power-supply-750w-110-240v-80-gold-92-efficiency/ At 750W, it could support 3 to 6 Apollos, depending on how you run them. On ECO mode 5 would be pretty safe; on TURBO it could handle 3 with a good margin of safety. You'd have to measure your power draw, and do some math to be sure.