Well, composers, conductors and improvisers (e.g. jazz, folk) need to have it, so that would put it to several hundred thousands, not "less than a thousand".
Maybe, but I think improvisers wouldn’t even have to be able to read music at all. Just imagine sounds that go with the current tune and then play them. You don’t have to picture a score in your head to do that. Whenever I’ve jammed with other people that’s how it’s worked.