i dont think there is any limitation of connections .SQL Server will handle large numbers of connections without a problem. A more important question is whether your database schema and application code will be able to handle it.

More users trying to access and modify the same data at the same time will likely lead to a lot more request blocking from all the table/row locking in the database. The performance of the app may slow to a crawl with that sort of usage. Without seeing your database schema, it's hard to tell. Although assuming you just did a simple conversion from Access to MS-SQL, it's likely you're not taking advantage of things a real RDBMS is good at like constraints, foreign keys, and indexing.

FYI SQL Server allows a maximum of 32,767 user connections.