The Q Continuum is an extradimensional plane of existence inhabited by a race of beings known as the Q. The term could also refer to the Q society itself. Per Memory Alpha we're told of 8 individual Q plus the addition of Riker's temporary promotion to Q status brings the total number to 9.
The question is;
If they're an actual race, how many Q are there?
Based on the events of "The Q and the Grey", there are potentially hundreds of Q. In the battle scene we see scores of Rebel tents and tens of campfires. Assuming that each Q has their own tent, then just one side has approx a hundred Q. One would assume that the forces of the status quo (e.g. who're winning) would have at least that number again, maybe more.
I'm going to interpret this question as 'How many Q do we know about', as otherwise we'd be merely speculating.
We know of a total of 20 Q:
That is not to say that there are only 20 Q including Riker; it's just saying that we only know of 20 being who were a Q
