Have you ever gone to buy a shirt and found that the medium in one t shirt is not the same as a medium in another brand of tshirt? That's the same situation with reeds and it can make you incredibly frustrated at times! Reeds are pieces of cane and when they are wet they help you make the most beautiful music on the saxophone. Finding the perfect reed is the quest of every saxophone player I know...even me!
Every brand of reed has its own saxophone number system. One important point to keep in mind is that what works for you is special to you. So, don't worry if you play on a 2 and your friend plays on a 3. The number of your "perfect" reed is determined by the mouthpiece you are playing on and your own special mouth.
When you are starting out, ask your teacher to help you find a reed that works for you. I often find that Rico Reeds work great for beginner and intermediate players in a strength of 2 or 2.5. But, ask your own teacher. We all have our own favorite reeds.
What do I play on? I actually play on a few different brands and I'll write that in a different question soon.