If you smoke only 1 to 2 blends and don't dedicate I'd say 3.
If you like a really fresh pipe like I do then at least 7, one for each day of the week. It all depends how often you smoke too. If you're an all dayer 2 pipes won't really cut it. They won't get a chance to rest and you won't be cleaning it as much as you should and you'll lose flavor and aroma .