Well, that depends entirely on how much data each user consumes. A small server with three SCSI-1 disks, which can serve about 960KB/s of data, will support between 36 and 80 clients in an ordinary office environment where they are typically loading, and saving equal-sized spreadsheets or word processing documents (36 clients 2.3 transfers/second 12k file 1 MB/s).
On the same server in a development environment with programmers running a fairly heavy edit-compile-test cycle, one can easily see requests for 1 MB/s, limiting the server to 25 or fewer clients. To take this a bit further, an imaging system whose clients each require 10 MB/s will perform poorly no matter how big a server is if they're all on a 10 MB/s Ethernet. And so on.
If you don't know how much data an average user consumes, you can size your Samba servers by patterning them after existing NFS, Netware, or LAN Manager servers. You should be especially careful that the new servers have as many disks and disk controllers as the ones you've copied. This technique is appropriately called "punt and hope."
If you know how many clients an existing server can support, you're in
much better shape. You can analyze the server to see what its maximum capacity is and use that to estimate how much data they must be demanding. For example, if serving home directories to 30 PCs from a PC server with two IDE disks is just too slow, and 25 clients is about right, then you can safely assume you're bottlenecked on Ethernet I/O (approximately 375KB) rather than disk I/O (up to 640KB). If so, you can then conclude that the clients are demanding 15 (that is, 375/25)KB/s on average.
Supporting a new lab of 75 clients will mean you'll need 1,125KB/s, spread over multiple (preferably three) Ethernets, and a server with at least three 7200 RPM disks and a CPU capable of keeping up. These requirements can be met by a Pentium 133 or above with the bus architecture to drive them all at full speed (e.g., PCI).
A custom-built PC server or a multiprocessor-capable workstation like a Sun Sparc, a DEC/Compaq Alpha, an SGI, or the like, would scale up easier, as would a machine with fast Ethernet, plus a switching hub to drive the client machines on individual 10 MB/s Ethernets.
