You can run multiple klipper instances in parallel if you use our installer. They are separated independent installations. Klipper runs on the pi and printer. CPU intensive stuff on pi, io stuff and stepper control on printer board Also marlin in addition is no problem. How many I can not say. Depends on pi and memory and speed of klipper whcih depends on python version, klipper version, .... It is only limited by cpu power and memory in theory, also multiple printers in parallel can cause electric issues in some cases.