I've worked at places with two developers and 150 repositories. I've worked at places with 12 developers and 5 repositories. Who do you think gets more marginal utility out of another repository?

Every place I've worked that hasn't used GitHub either did so because of a regulatory reason (HIPAA and not wanting to do on-premise) or because the per-repository pricing model made it absolutely stupid expensive, or dumped you straight into Enterprise.

If you make a lot of money off a single private repo with many users, you'll move from "so cheap you don't even need to think about it" to "market rate." If you make a little money off of a lot of repos with a handful of users, you'll move from "don't even consider it due to cost" to "let's try it out for six months and see how it goes."