Number 1 is the thinnest and they go up in 1/2 sizes. 1, 1 1/2 would be the best bet for the first timers. You could move up to a 2 in not too long. As soon as you’re able to play steady tones and move around all the notes etc. You will notice the slight difference from size to size once you can get steady sounds. From there it’s up to you because it depends on how much harder the 2 is over the 1 1/2 etc.
But Don’t Make This Mistake…
Some beginners will only buy a few reeds at a time. In a box of 10 we’re lucky to get a few top notch reeds, so if you buy from a music store that sells singles from a box and you pick up 3 crappy reeds then this is all you have to go on. Buy at least one box of 10, soak them all and blow thru each one for a couple minutes… this is how you’ll get good at knowing if a reed feels good or not.