Normatively, any city parish would have had a large number of acolytes, subdeacons, at least one deacon, and probably three or four priests. Some churches would be run by canons, which were a decent sized number of priests (in a religious order or given funds by a bequest) dedicated primarily to singing the Office and the Mass every day. So, say ten or twenty singing priests. Also, many churches would be staffed by orders of monks or friars, so even if there were only a few priests, there would be tons of guys singing the Office and the Mass every day.
At the cathedral, which usually had seminarians as part of the bishop’s household or in the seminary next to his house, part of seminary training was learning to sing the Mass and the Office. If you had a cathedral school attached, they gave boys a good education in exchange for using them as choir sopranos and altos; and of course, a kid who already knew how to sing the Mass and the Office was halfway to becoming a priest, if he decided to go to the seminary. But again, you would be practically crawling over clerics with singing voices. (And a good singing voice was a sign you might have a vocation, while a bad singing voice made it less likely.)
But smaller parishes would usually have a priest or two, or a deacon, or someone who could be a cantor. So you didn’t need a choir if you had a cantor.
We are experiencing a bit of a priest, monk, and friar shortage at the moment, which is why the idea of having a fully staffed church with a whole schola of clerics seems weird to us.
