Bill,
It's NOT number of arrows!! It IS when you start to feel it to stop. Using the rotational draw, you are learning to use bone leverage rather than muscle to draw the bow. But as you work on technique and up in weight, you WILL feel it in your back -- that is when you MUST be honest with yourself and stop.
As you learn and get proficient, your stamina will increase but in my opinion, this is not like lifting weights. Don't continue until you feel the "burn" that means you went too far. You can press your limits a LITTLE with a lighter bow but with the heavier DON'T.
Shoot until you know you are tiring then stop! You will be surprised how fast you will increase stamina (number of comfortable shots). Also, if you are jumping significantly in bow weight from your "training" bow to a heavier bow (10# is significant) shoot each one every other practice session WITH a day or so of rest in between.
Arne