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Max connections SQL Server Express could have?

I have sql server 2005 express edition installed on one of my client's machine, and lots of other users are also connected to that sql server through the c# application that I have provided them as an interface.

Sometimes a pc get connected to sql server and sometimes just throw the exception "SQl server not found....".

I believe express edition reach maximum connection limit of it and then gives error.

Please guide me how could I solve this issue.

Any white paper that help me know which edition have how many number of parallel connections,

I am open to upgrade to sql 2008 or sql 2012, but prefer express edition. I could go for other versions as well if they really pay me back in this context, otherwise express edition is doing well.