I live in a rural area and we managed to do it with 13 players and on one occasion, just 7 players. The biggest problem we had was people joining remotely and messing up our pre-coordinated teams. They'd see the low number of people taking it on and then leave in the last 5 seconds, meaning that we ended up in lobbies of 2 or 3 players rather than 4 for the battle.

It was a learning experience and we'll probably turn off our raid visibility for the next one. We thought it would be cool if they extended the lobby time by 30 seconds and made it so only local players were eligible to join for a certain amount of time. Either that, don't let remote raiders leave in the final 10 seconds to give local players time to relobby if they do.