What reed number should I use?

I have played for 6 years. I'm a high school sophomore and I am getting pretty good. I play a medium soft (I believe it is in the 2.5 - 3.5 range), and I'm getting new reeds today. I want to sound good, so what should I pick?

Alto Saxophone

It depends on the mouthpiece, not on how long you have played. This is a myth that for some reason continues. With a mouthpiece that has a small facing (space between reed and mouthpiece tip) you want to use a hard reed. If you have a mouthpiece that has a large facing, you want a soft reed.
Below is a link showing a lot of mouthpiece facing info and should have your mouthpiece on it. For most of the mouthpieces that come with saxophones, what you are playing is fine, if not a little hard (3.5 is pretty hard). Also, are you getting a good sound from your saxophone? Is it too spitty? Too reedy? If not, get the strength of reed that gives you the best sound.

you didn't say what instrument you play, but for clarinet you definitely should stick with the range you're in, a 3 is usually good. reed thickness also varies with brand, and that does make a difference. I believe it is vandoren that is thicker than most other brands. honestly, you should really be asking your band director this question! especially since he or she should know what skill level you're at, what mouthpiece you're using and what kind of instrument you're on--all of that makes a big difference in your sound!

people usually start on a #2, and i've been playing for 3 years and i use a #2.5, but i would say if your pretty good then #3