One highly effective one would be the minimum although more would be much better.Don’t we only need one to work?
I was thinking more of the mental effects if one of the main candidates e.g. the Oxford vaccine failed before we had a success. Even though that wouldn't be unexpected and wouldn't doom attempts to find a working vaccine we already have people talking about giving up and letting the virus run wild. The current poor levels of compliance we are seeing in places would probably escalate and that could be disastrous.