Quick question, is there a limitation on the number of users that can get into SQL server at one time? Say 1500 or 2000 users? If not, I wonder how many users can actually cause the performance of the server to have a significant drop.
There is a pretty hard limit at 32000 users, but I think it could be broken given sufficient hardware. I'm not sure that it makes any sense to even go there, other than as an academic exercise because it smacks of poor system design. A federated replicated approach allows practically infinite growth from an application perspective.
Most servers will crunch LONG before you even begin to approach the limits of the database engine. The number of supportable users usually depends on the RAM use more than anything, although the I/O bus often factors into that decision too.