This topic came up a lot in SBA 2006 and I see it still causes some misunderstandings in places. So I thought I'd blog about it here and link to it in future news group postings.

There is no preset limit to the number of users in Office Accounting - concurrent or otherwise.
The old MSDE (used by SBA 2006) used to have a built-in performance degration when more than 5 concurrent connections were used (concurrent workload governor - a performance throttle). Note that even that did NOT translate into a maximum number of users - you could have an unlimited number of users and only if more than 5 just happened to use a connection at the exact same time would it result in a performance degration.
But there is no such limitation in SQL Express 2005 - instead they have created other limitations like; never use more than one CPU, max use 1Gb RAM, and max size of each database (4Gb).

Here's a link to a page explaining the limits of SQL Express 2005 :
If you want to overcome those limitations you can easily use the full SQL Server 2005 instead.

In fact Office Accounting should scale very well since it relies on Sql Server - but you might find that if your company grows to more than 25 employees it could require more advanced accounting software with more features than Office Accounting.