Correct, the number of process instances that are currently active isn't all that important, as they are only loaded into memory if they need to move forward. Having thousands of process instances managed by a singleton session is possible as long as that session is capable of handling the load (i.e. the requests per second that are necessary), as typically it only means that there are thousands of rows in the database (one per process instance) but only a few of those will be in memory at all times.
For example, if every process instance only changes typically once a day, then one session can handle that. If they all change every second, you'll need to distribute your requests amongst multiple ksessions, as one simply won't be able to handle the load.
Kris