Suppose a wireless router has a bandwidth of 54 Mbps. Does this mean that 2 clients (PC's) that are connected to the router can both transfer data at 54 Mbps simultaneously or will the bandwidth be split equally among them (27mbs each)?
The quoted wireless bandwidth is the total bandwith for that the AP can provide, and is half duplex.
So the AP can handle no more than, for example, 54mbps of data. It doesn't matter how that traffic is distributed, 54 clients each sending 1 mbps or 54 clients sending .5mbps and recieving .5mbps, or 1 client sending 54mbps or 2 clients, 1 sending 10mbps and recieving another 10mbps and another sending 24mbps and recieving 10mbps etc...
This is a rather naive example, in practice very few wireless networks can ever achieve their full quoted data rate. It is strongly dependant on signal strength. There are then various overheads, for TCP, IP and the wireless transport layer, including traffic that manages the connection even if you are not actively using it currently. These overheads include acknowledgments that need to be sent when data is recieved (and vice-versa).
Last thing is that some APs suffer badly when they are required to deal with mixed clients. My previous Netgear was a rangemax 240 which THG found dropped total throughput to less than 20mbps if a rangemax client and a standard g client connected at the same time. There are similar issues with supporting g and b clients, and although I don't know for sure, I expect that similar issues exist for supporting n and g clients.
The more users the less bandwidth each gets. Exactly how much will depend on exactly what is happening on each client. If both were downloading a file at the same time, there would be less than if each were just browsing pages. The download is more of a steady steam of data while browing generates the page dowload and then a period of little activity while the page is viewed. Then another spike when a new page is downloaded. This is a simplistic model and it is more complex but think of it as a pie. The more slices you need to make, the less each person gets.
It really has nothing to do with the number of "users" or wireless clients. If I'm downloading a 1gig file at 1mbs that would leave 53mbs (on a 54mb link) for other applications to do stuff. However, if I'm able to get that 1gig file with a rate of 45mbs that would leave 9mbs for other applications. The speed of the flow depends on bandwidth, routers, network conditions between the two devices involved in the flow.
Note: due to header overhead etc, you will never get 54mbs of transfer rate out of a 54mbs network.
And it's really not that straight forward, but it's a more accurate way of thinking about the problem.