In small Apt. living...gotta be enuff.
Guess this Q applies only to homeowners/ Whole House dwellers.
Yes and no.. as someone who lived in apartments with roommates, girlfriends, alone, and in various sizes of places, I found 1 system to be adequate for an apartment... and it was always in 'my' space. So, in a shared place, my system was in my room... If I lived alone or with people I could (sort of) trust, in my den, where I could sit in a comfy chair. However, I always had a car and portable system during these times too... (I lived in the south, with limited public transport where you can drive a car for 1,k a year.)
So, 3 is the minimum if you have a car, 2 if you take the train or bike-commute. So, that's 'enough' to enjoy music. However, what's enough to explore all the different kinds of sound you want? You can certainly setup quasi-multiple systems in one place... so, experimenting with new speakers, amps, etc... you can have 2 system's worth of gear in one place easily.
I got tired of the cramped spaces and limitations of rooms so I changed jobs and moved somewhere I could buy a house where I could have multiple systems. It's fun to build them. It was a good choice. I don't have the nice city/community feel I did in Boston, DC, or other cities I've lived.. but that's the only thing I miss... there is so much more that I'm happy to miss with this choice...