Why not set it to 250-300? That way you'll always have enough connections.
Okay, that's a snide answer. But seriously, why even worry about setting an upper limit?
Well, the answer has less to do with your users, and more do do with workload and througput. The real question is, how much can your database handle. If you database can handle 300 connections, put it to 300. Does performance degrate before or after 300? When do wait times become unacceptable. Does that happen after 10 connections or after 30 connections?
My idea about connection pools is that I limit them to the point where creating the nth one no longer brings any performance gain. After that point, you either limit the number of connections, or increase your database capacity.
What about licenses? Are you capped by licenses?
These are all some of the redirects I'm trying to give you to avoid the entire topic.
Here's a little something on my website that provides more information about configuring a WebSphere 5 or 6.1 connection pool or datasource: