First find the number of feathers Milly can get from one flamingo: 20 feathers * 25% = <<20*25*.01=5>>5 feathers
Then find how many feathers she needs total: 12 boas * 200 feathers/boa = <<12*200=2400>>2400 feathers
Then divide the number of feathers she needs by the number of feathers per flamingo to find the number of flamingoes she needs to harvest: 2400 feathers / 5 feathers/flamingo = <<2400/5=480>>480 flamingoes
#### 480