There is no defined limit on how many different users can virtually access your tables concurrently.

Bearing that in mind, remember, in REALITY, only ONE user at a time can lock the table to perform updates. This lock can be a record-level or table-level lock. If you will have concurrent access to a table you'll need to allow for it in your code by controlling the type of locking used, retry intervals, etc.

See online help Chapter 17 "Programming for Shared Access" for more details.