It's really a personal decision by which criteria you apply. If the job seems absolutely perfect, 15-20 would not deter me. But you never know how far the client will scroll down on his list of proposals.
I think it depends on whether you have something to set you apart from the crowd. I've been hired on jobs I bid on after there were dozens of proposals, but they were always firmly in my niche, where it was unlikely that more than a handful (if any) of the other freelancers had comparable credentials.