You really need 3 wrenches:
13/15
13/14
15/16
17mm is rarer but it does show up every once in a while on the outside locknut and often you can get away with using a regular 17mm as long as you are not trying to adjust it in the frame.
Often you might need two 15mm wrenches at the same time, or a 14 & 15, or a 13 & 15. Having those three different wrenches I listed above will cover you for most situations. I don't think I've ever run into a 14/14 combo or 16/16.
If you only have the 13/14 and 15/16 and you run into a situation where you need both 15's or both 13's you can't use both at the same time without cutting the wrench in half. The 13/15 comes in handy.
If you buy them all on amazon at the same time you can combine shipping (which runs nearly as much as each wrench itself.)