Re: milk supply
If you really empty the breast, you could be making the flow slower at the next feeding and that can make baby fussy. But don't think of this as a big deal- baby will still get fed, he's just going to have to work a little harder to get a full meal. And that hard work is valuable, since lots of intense suckling is great for building supply.
S to how many letdowns you "should" have during a feeding, it depends. Some moms can easily generate multiple letdowns when pumping, others get just 1-2. If your supply is well-matched to baby's needs, 1-2 letdowns is very normal.
Coolest thing my big girl said recently: "How can you tell the world is moving when you are standing on it?"
Coolest thing my little girl sang recently: "I love dat one-two pupples!"