To my relative noob-ish (4 years into pipesmoking) opinion, it al depends on your smoking habits; frequency, blends, smoking technique (wet/dry, pace, do you let the pipe get hot), prefered material and how much of a horder you are. Those factors are highly debateable, not starting a discussion about ghosting and drytime of the pipe here as I dont think I have any evidence other than my opinion. I think someone who smokes goopy aromatics wet as a drying towel after doing dishes might need more or different pipes than a all virginia smoker which keeps an infrared thermometer at hand for checking the temperature of the pipe every minute. Those are extremes ofcourse, but just a indication how different we can be.
I have 10ish now. I dont need more and could even do with less, as I could smoke my 2 favorite meers 80% of the time. With my meers I dont mind smoking them more than once a day or even back to back, where 2 bowls is the most I smoke at the moment. Im not a big aromatic smoker (here and there a vanilla one like MB spuncuts), I do like english mixtures, so maybe 2 for englishes and 3 for the rest. But then again, everytime I see how Castello makes a chubby pipe look great (and I like long pipes more) the PAD itches.