How many applications can I have open at a time? Is it reccommened?

I have been playing the Witcher 3 a lot and have been having fun but I often change between the witcher and say Amazon video or netflix or something. Is it okay to keep the witcher running in the background (just pressing the ps button) and navigating to amazon video or how does all that work? How many open applications can I have and is it bad to have too many?