You probably will not know these numbers until your proxy server has been running for a while. Until then, you can use a typical proxy server setup. Table 1.1 describes the hardware in a typical proxy server setup. A proxy server deployed on a UNIX machine can support approximately 3,000 users. This estimate assumes 300,000 requests per day under standard conditions and depends on the hardware you select.
Concurrent users are generally an order of magnitude less than the total number of users.
It is critical that you have enough actual RAM to hold all the processes in memory when they are active. Table 1.1 shows the minimum amount of RAM you will need for your proxy server. You will generally need more RAM as your user base expands. Table 1.2 shows RAM sizes based on the number of users going through your proxy server. Large deployments should also consider a logging file system or non-volatile RAM to allow the server to perform asynchronous writes to the disk.