20 developers is a lot, but could in theory be worthwhile. I'd really try to get all 20 be very-effective ones and structure the responsibilities and communication so that they don't get bogged down. If you do it a naive way, and aren't lucky, it'd be easy for 20 1x..10x developers to become 0.01x developers.
Or, if you get people who just want to call themselves a "dev" and grind concrete task assignments in a straightforward way, then I think you either need to be doing something bog-standard and naturally amendable to just going through the motions (e.g., scalable Web CRUD, or yet another forum/chat app), or you need a brilliant approach to structuring the work.
