There aren't any "hard" rules about how many devices should be on a subnet or VLAN. The main reason is that there aren't two networks just alike. Each one is different and should be evaluated on it's own merits.
Here are some steps to take:
1. Gather information
a)protocols - which ones are being used
b)applications - how many?; how "chatty" are they; what protocols does they use?
c)devices - how many?; which apps on what devices
d)network - shared?; switched?; operating limits of the switch itself?; protocols supported by the switch; and so on.
You will need some sort of protocol analyzer to examine traffic patterns and determine what protocols are running. This will also help you in determining what percentage of your traffic is broadcast and the current utilization.
The newer switches can handle an amazing amount of traffic (forwarding rate) while offering features like the ability to limit the amount of broadcast traffic. In my network most of our VLANs are either /24's or /25's but we do have one /23 (510 devices). The /23 is a VLAN implemented on a Nortel Passport 8600 and has absolutly no problems. There are currently only approx. 325 devices but with past performance, it should eaisly handle 500. We run only IP (a BIG factor-single protocol) and a lot of different apps. Any good switch (we also have Cisco and Extreme) should be able to handle the traffic.
From a design standpoint, I try to seperate my networks by purpose, with a general rule of thumb as to the number of devices at /24.
One of the main advantages of a VLAN lies "hidden". In my opinion, one of the biggest benefits is the lower administrative cost associated with AMC, (adds,moves,changes). Very seldom do we have to change any cable, just add the port to the VLAN, or remove if that is the case, and your done.
