Few questions raise quite so much debate within the pipe smoking community. Most will advise that you should have enough pipes to allow for adequate rotation, whereby each pipe gets at least 24 hours rest time between smokes. This is to allow the moisture that the pipe absorbs from the burning tobacco to completely dissipate and dry before you smoke it again. Smoking a pipe that hasn't properly rested can result in a sour taste, sometimes enough to really spoil your smoke. Note that this is more of a consideration for briar and other pipes made from wood. Meerschaum pipes and cobs require less resting but in any case it's nice to have a selection so you can choose the piece that fits your mood. It's also worth commenting that if you tend to smoke a number of different blends you may want to reserve some pipes for particular types of tobacco. For example, one pipe for heavy Latakia blends, one for aromatics, one for Lakelands, whilst still keeping enough in your collection to allow for rotation. Having 'blend specific' pipes relates to the tendency of some tobaccos to ghost your briar (leave a residual taste and aroma which will carry over to your next smoke). As mentioned in our introduction to pipes, meerschaum and cob pipes don't ghost and the same is generally noted of morta pipes (bog oak).
This does beg the question of how pipe smokers fared in the past when it wasn't unusual for a smoker to use just one briar pipe day in, day out. The truth is that many report you can happily manage with one pipe if you clean it after each use and give it an occasional deep clean. However the general consensus does advise having several pipes available to allow adequate resting and the best possible smoke every time.
Of course in answering the question of how many pipes you need, we'd also need to factor in how often you smoke. If you smoke once a week then one pipe is surely enough. If you go through several bowls a day, every day, then it would be wise to have several pipes available. Beside 'best practice' or practical considerations you'll probably find, like most of us, that you want at least a small collection to indulge your preferences for different styles or materials, so for this reason alone most pipe smokers will build up a collection over time.
If we only consider the question from a practical viewpoint then we'd suggest having a minimum of 3 or 4 pipes in rotation. The most common number you might hear is 7 pipes, the idea being one for each day of the week.
So is there a definitive answer? In short, no. Buy as many or as few pipes as you feel you need, or perhaps better said, as many as you want. Chances are that you'll anyway soon succumb to the dreaded PAD (Pipe Acquisition Disorder) that many of us develop!
A selection of pipes by Dunhill, Paronelli, Peterson, Northern Briars and a Turkish Meerschaum