As far as "how much" there is only one real metric: the minimum number of commitments. A proposal needs at least 200 people committed to use the site in order to succeed at the second phase. So if a community is smaller than 200 people, a Stack Exchange site is simply not for them yet. We have closed proposals where we have noticed the listed community seemed to only consist of a dozen or so people. They need to do a bit more growing first. Past 200 people, there's really no gauging anyone does here. It's up to you and the community to ensure enough people come over and participate in the process.
Is that enough evidence of who would be contributing to a site, or do I need to get at least one of those groups to commit to contributing?
This isn't the place to create proposals just because you, personally see a benefit in having a Q&A site for a specific community. This is definitely something that should be brought up and discussed within whatever community or organization to see if those people would even be interested in a Q&A site.
I've seen countless people list subreddits as their community source, and they never go anywhere. They oftentimes just link it as evidence of an existing community without even bothering to advertise it there in any way. But you also have to consider that a community that seems to only exist on reddit is unlikely to want to move to an entirely new place or split all of their discussion between two places.
Ask what problems you are solving by creating a Q&A site. Are there gaps in participation that would be uniquely filled by our platform, or are you coming here simply because you think it's a cool idea? Would participation benefit from having a Q&A platform that disallows more discussion-y type questions, or is the existing community focused mostly on discussion and would not benefit from a Q&A format?
No matter how much evidence you have of an existing community somewhere, that is not enough to launch a site. Without the support of that community, you are likely wasting your time. Don't propose new sites and then find out that no one there is even interested in it because you never asked them first. Ask them and then, if they say it's a great idea, propose it to make it a reality.