How to decide the parameter processes count?
Yes, it depends on your user count. But not only that, it also depends on which way your users connect to the database and the number of background processes your instance requires.

There are basically two types of server processes that services user processes (client processes): dedicated server process, where there is an one-to-one mapping between user process and server process, and shared server process, where, like the name suggests, a server process can be shared between two or more user processes.

If your application (client) connects directly to the Oracle database, it will spawn a dedicated server process to handle its requests. If your application connects to a dispatcher, then you are using one of many shared processes to handle your requests.

Basically, if you use shared server connections you can service many users with fewer processes, but you may have a performance penalty. More information about dedicated and shared server processes can be found at: https://docs.oracle.com/cd/E11882_01/server.112/e25494/manproc.htm#ADMIN11166

Also, please beware that what prevents you from connecting to an instance is reaching the SESSION limit, which is a derived metric from the number of processes:

sessions =  (1.5 * PROCESSES) + 22


Since every login requires a session, it's the sessions parameter that actually determines the number of concurrent users on the instance. Please refer to the following url for more details: https://docs.oracle.com/cd/E18283_01/server.112/e17110/initparams229.htm