Unfamiliar with networking, so here goes...

A new (potential) client has several computers on a network, and will be getting FMP (in some version) to run a solution over the (Wintel) network.

How many computers should be linked before it's worth buying the Server version, how many can run on the one copy of FMP Server, and what else (if anything) is needed?

It would be nice if the FMP website were more informative. Ah well.

10 should the absolute MAXIMUM before moving to Filemaker Server. However it also depends upon the use and the value of the data. Filemaker Server is more secure and stable, so that is a huge benefit even to 3-5 users.

Filemaker Server can host up to 125 databases and 250 simultaneous users. For bigger solutions or installations, multiple servers can be used. One client had 4 servers and more than 250 users across an entire state, another client has only about 25 users, but over 500 files across 5 servers.

Get the Filemaker Server best practices white paper from Filemaker's website, and follow it! Basically Filemaker Server ONLY, no e-mail, no web, no filesharing, no DNS, no Domain Controller, NOTHING but Filemaker Server. This really applies to any database server.

I agree 100% with the above.

There are some exceptions, which are working, but they do not make new rules.

IMHO FM client as server is good just for the testing purposes.

FM server will protect the data and will do automatic backups etc.

FM Client is OK for testing some things, but it will be misleading for testing actual performance issues with FM Server. I always recommend that my clients get on FM Server as soon as is possible, even long before the solution is rolled out to the users. Both the enhanced team development and real performance tuning is essential to a good development project.

Sure.

The performance of server is 10 fold of that of client.

It is just it they don't want to spend money without testing functionality of the solution.