Characterize the resource requirements of a typical user with Performance Monitor, network analyzers, or by other means. The most important resources to monitor are CPU, memory, the paging file, disk subsystems, NIC, and LAN or WAN bandwidth utilization.
Based on the above guidelines, you should be able to arrive at the optimum server configuration (CPU, RAM, disk, NICs, protocols, services), including the number of servers required, the number of users per server, and the network design considerations (backbone segments, multiple NICs/protocols).