I smoke about 5 bowls a day. I like a fresh pipe, so 5 pipes would probably be enough. Except that I like to smoke while I am cleaning them, so a good rotation would be six. I have 114 of the things, lol. Overall, I have probably owned more than 150, over 40 years. Lost a couple, gave a number of them away, broke 2 beyond repair, and sold 4. Have 7 pipes on various wish lists. Many of them that I do have are attached to memories, making it hard to part with pipes, even those that I haven't smoked in years. Recently, I decided to smoke each one, and get rid of the ones I didn't enjoy. Well, no luck.
So, you only need one. For me, a good rotation would be six. I have a lot of respect for someone who can stop at six.