MichaelRed (who's information I have found to be pretty reliable) has claimed to have 75 concurrent users. Unfortunately, I wasn't able to find the thread right now, but if I find it later, I'll post a link.
Others have warned of difficulty around the 10 or 50 mark. I have only personally tested with 3 concurrent users, but I expect to test a larger group in the near future.
When SoloDroid said the limit is 25, I think Microsoft claims the actual limit is 255. This is probably a hard limit, beyond which the software cannot support users. However, I'm guessing that there exists somewhere a practical limit. I'm personally hoping with my database design that 100 would be permitted. I'm not expecting that many (and would actually have difficulty testing it), but that is my target number.
Please let me know how things work in your environment. Remember that database design plays a significant role in the performance, so 10 users might be too many in one design, while 50 might cause only trivial impact on others.
Good luck to you.