There is a persistent myth that Microsoft Access Jet databases can only support 20 or so users. Here's my response to a recent inquiry:
I question the data for the limitations on the number of Access users being around 30. We've run many tests and have never seen that kind of degradation in performance. It's a myth from Access 2.0 days that was eliminated with Access 97 over a decade ago. A well designed Access database can support hundreds of users. Of course, what matters is the number of simultaneous users, and what they're doing.
If everyone is just viewing data or entering data into a table, that takes very little work and a large number of people (well over 200) can be supported. People cannot type faster than what Access can handle. If they are all running massive reports and queries with data updates, that can still be done but performance would be an issue which applies to any technology, so testing and optimization would be necessary.
If the back-end database is in SQL Server rather than an Access/Jet database, the number of users can be practically unlimited if each user has their own front-end copy of the Access application. Performance issues still apply based on what they are doing. In some cases SQL Server is slower than Access, so it is important to understand the situation before thinking SQL Server is the answer.
All that said, any Access application that is distributed to others with shared data should be a split database design. Here are a few resources we've written:
We also offer a commercial product for enterprises, Total Access Startup, that helps with the distribution of databases to each user's desktop and launching the right version of Access.