You could say "minimum number." But, unlike Robert B,I don't think there would be any problem at all using "quorum." It means "minimum number of people required to do the business at hand." I only mentioned, previously, that it is usually found in the context of business or government, but it fits in this situation as well.
Yeah, I subscribe to Dick's definition. When I wrote the constitution for our astronomy club, I described our decision-making process as follows:
Regardless of the number of members in attendance, those present shall constitute a quorum for voting on all matters of club business.
I wrote it that way because we're a small club and often need to make decisions quickly. During our "constitutional convention" everyone knew what I meant and passed that paragraph without discussion.
I'm a little uncomfortable with saying that quorum merely means "minimum number". The only context I've ever heard it in has to do with authority to take action, as by a sufficient number of the members. Thus the bylaws may degree that a meeting of the board of directors cannot pass any measure having to do with issuing stock unless a quorum is present. If I wanted to say that I wouldn't teach a class unless at least n students signed up for it, I wouldn't use "quorum" for that except jokingly; the concept is similar, but not the same.
In Glenn's defense, I think we can legitimately make a distinction between its careful and correct meaning and a deliberate misuse for humorous or metaphorical applications. As we used to say in my family regarding some table manners (not all), "it's ok as long as you know better". At the dinner table that meant "I won't complain about you doing that here at home, but if I see you doing it while we're eating out we're going to have to tighten up the rules here too until I know you can eat without doing it that way." It seems to me something of that sort can be said about the misuse of certain words: As long as you know it's technically not the correct one, I wouldn't complain about its use in this context just to make the point clear. But if you end up confusing the point rather than clarifying it, or if you appear to think it's actually the right word, we may have to have a talk.
Of course, maybe we'll have a talk anyway, just for fun. That is after all the whole point of this forum which Grant and Martha have so fun-lovingly provided for us.