I think there is no correct answer to this question. More personable than anything! I have 30 locomotives. I decided that would be the magic number. Can't buy a new one unless I get rid of another. Having enough for your layout to operate, plus a couple is, I think, a good way to judge the correct number.
The other obvious answer to that is when it starts to hurt your finances, you have bought too many!
The SIW has 7 operating locos 4 geeps 2 GE 70 tonners that usually operate as a consist and an 0-6-0 dockside steamer thst usually appears at the holidays.
I can only run maybe two locos at a time and like having one or two others idling.
So my question is how many locos do you own (not countin some non dcc shelf queens )and how many do you actually run?
Joe Staten Island West