This won't tell the congregants whether there's confirmed minyan
Right. Now theoretically (I haven't tried this to know if it works) you could have a second "zap" which added people to a google spreadsheet when they reply, and a third "zap" which, whenever that google spreadsheet is updated, checks if there are ten entries and then sends out another SMS to everyone.
You're kind of reinventing the wheel, though, so if something like twilio (posted above but I don't know anything about it) can do that, you may be better off.