I don’t know about this, specifically.
Generally speaking, assuming your app is distributed through the app store, there is effectively no download limit. If you distribute your app by other means, that other means may impose download limits, but that’s up to you to determine.
AppSheet itself doesn’t impose user limits. Depending on your data, 5,000 users all interacting with the app at the same time could cause performance problems. There are so many variables involved, though, that it’s hard to say.
That depends on your data design. AppSheet apps work with a copy of the data. Changes made by the app are only shared with others when a sync occurs. The app does not sync in real-time. Consequently, one user won’t be able to see what another is doing in real-time, and may even need to wait minutes, tens of minutes, or even longer (depending on how your app is designed) to see changes by other users.