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I figure enough for each to be smoked once per day is about right. Every pipe* rests a minimum of 24 hours in an area with circulating air. Then figure out bowls per day, factoring in any dedicated briars, should give you your answer**.
*Cobs don't count
**But you should probably get 2 more cobs and a Grabow just to be sure.
Well, my Social, Menu, and Financial Advisor gave me, for Christmas two years ago, a 48 pipe wall cabinet; quite nice indeed.
Her plan was I would fill that cabinet and thus limit my PAD.
The other day she said, you need another pipe cabinet. One of these days I'm going to attempt construction of a 40 pipe cabinet that will roughly match the other cabinet in size but allow more room for the pipes. I'm not sure that will hold all my pipes, but it will be closer!
In answer to the OP's question: As many as you want and can afford.
Falcon pipes, screw on bowl, aluminum body. Buy one complete pipe (new or vintage) and however many bowls you may need/want. A bowl for every day of the week and/or one for every tobacco blend you smoke.