How do you pick the clarinet or saxophone reed that is the right strength “for you?” You mostly don’t, really.
It’s important that the reed be a good match to the mouthpiece. In most cases the primary consideration is the mouthpiece’s facing curve and resultant tip opening. Generally, a shorter curve and/or wider opening require a softer reed, that can flex enough to meet the mouthpiece while vibrating. A longer curve and/or narrower opening need a stiffer reed, which will have enough guts to spring back after flexing to the mouthpiece.
This means that the “right” strength for a player using a particular mouthpiece will be pretty close to the “right” reed for anyone else using that mouthpiece.
Some players and teachers object to this, insisting that the “strength” of the embouchure needs to be accounted for. But the embouchure shouldn’t employ much “strength”—it should close just airtight (but not tight) around the mouthpiece and reed. If you are using your embouchure to muscle the reed around, then you might think you need a stiffer reed, but what you really need is a more open, relaxed embouchure. (If you feel like you will lose control by relaxing your embouchure, make up for it with powerful breath support.)
So, assuming a reed reasonably well-matched to the mouthpiece, and a correctly-formed embouchure, the only thing left to consider is personal preference for how much resistance is in the setup. A slightly more resistant setup is good for things like soft, gentle articulations and stable pitch and tone. A slightly less resistant setup favors crisp, immediate articulations and some pitch and tone flexibility. I find this acceptable range of reed stiffnesses to be small enough that I can usually find some softer and some stiffer specimens within a box of reeds that are nominally the same strength.
Some mouthpiece and reed makers publish information about which reeds match to which mouthpieces. If you find yourself straying far from these recommendations, take a closer look at your embouchure and your stability/flexibility priorities.
