Even 50 person seems like a lot. I think people tend to forget that there's a lot of room at the bottom. If you want to give your product away for free you need to accept that there'll be less revenue, unless you can pull off the neat trick of segmenting your market almost perfectly e.g. the only people who use the free version are people who wouldn't have paid you anyway and your marginal cost is zero.

Could Docker as a developer tools business managed with 10-15 people? Quite possibly. Especially if it didn't try to become a generic VM on other platforms.