how to limit process instances?

Hello, how to configure processes instance pool that a given process will be running with only 1 instance?

I have many processes executing on document subscription and a few called by scheduler. The question is how to configure process A - he must be executed once at a time, and same for process B (process B may be allowed to 5 instances)?

Is there any possible way to configure that, or that could be done only by self written code that will check and count process instances?

Do web methods have process instances pool configuration?

English is not my native language; please excuse typing error.

Parallelity is controlled by the trigger calling a service for execution. For processes this is Default - subscription trigger in the generated Process runtime. Property Processing mode defines if the processing is done serial or concurrent at all, Max execution threads defines the number of parallel threads.
If this is about business processes you also should check the part Quality of service settings in the PRE documentation. Setting Optimize Locally and choosing the same logical server (e.g. default if there are none additional defined) leads to the starting document being committed after the process is finished.