It’s already answered that there’s no limit on concurrent users.
Your app may be limited by number of concurrent write operations to a single document. That is, two writes cannot happen at the same time due to (strict) serialization, so they get queued and retried, but there are limits.
This means you can have unlimited users, but if you give all of them the power to modify the same document, and then they all try to do so at the same time… that will cause problems.