Are there any stock agencies (or microstock) that do not require releases from subjects?
Every single one of them, only difference is it will say editorial use only vs allowing buyers to use it commercially as well. Getty, AP, USPW, Reuters, etc. small minority of those images have model releases, however, the average person is not going to just submit to them and you have to be invited, be known, a contributor. Stock agencies are not often looking for random submissions, microstock sells a lot of royalty free crap so they want model releases so that anyone can buy them, limiting it to editorial won't help them much at all because the images you're referring to are probably not that marketable.
Also, is it true that a photo of people in a crowd does not require releases? If so, how many people constitutes a crowd?
No, it depends on the usage if you need a release and not how many people are in it. Unless it's a huge crowd and nobody can be recognized, that's a factor as well if the person or people can be recognized. Also of course depends on where you're from and what laws they have as some countries have some very odd restrictions out there, I assume US?
What about people in a public environment, such as on the street, at a baseball game, or on county property at a state fair?
Streets, parks, and some other places are considered public - a baseball game, say an MLB game, is not even if funded by the state and on state property - state fairs may even be on private property. Just because it's public, like a school or library, does not mean it's "public" per say and you can shoot where you want.
I'm going to try stock for the first time and I'm trying to figure out if any of my photos with people in them are in any way saleable.
You should know this before trying out stock, what kind of images are you talking about, what do you plan on - or envision them - being used for? A release is about how the images are used, not where they were shot which falls more into privacy laws.
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