I am inexperienced, but if it were me, i'd make a judgement call based on scaffold and branch thickness. I'd thin to 1 fruit per cluster and keep fruit off of the end of branches to prevent the branches from being pulled down too much.
Newtown Pippin is a large appled IMO for B9, so, I'd definitely start with 1 per cluster with that. Smaller fruit, you could get away with 2 per cluster.

I think a good rule of thumb is to thin down to the king apple in each cluster, and allow no two apples to be so close that you can touch them both with the same hand.

This is hard for me to do- I don't like to take them off. But you really need to, and the sooner the better.

Apples are pretty supple; I doubt you'll have problems with limbs breaking if you thin to one per cluster. But thinning further is good for other things: easier to bag if you bag apples, larger apples, less tendency on the part of the tree to biennielism, more growth going into the tree instead of into little fruits, sprays coating the fruit better if they don't touch, and so on.

Good luck,

M

Your instinct is right. Thinning is even more important for a very young tree. One per cluster may be good for established trees, but a younger tree fruiting for the first time shouldn't be asked to bear more than a couple fruits on the smaller branches.

I agree with all the comments above. Just as a rough guideline, for an average vigor tree, I think it's safe to let a 3-year-old tree produce roughly 15 apples without stunting growth. But if the tree seems less vigorous or sort of a weakling, then bring this down to 5 to 10 apples. In no case would more than 20 apples be appropriate for such a young tree unless perhaps it had already been growing for several years at the nursery before you transplanted it.

These are 3 year old trees on fully dwarfing root stock. If they've grown well they should be able to handle a normal crop with an apple every 5 or 6". It really depends on the relative vigor of the trees but in commercial orchards they would be in production mode by the third year.

Remember too to look for damaged apple first to remove.

Also start now to keep your orchard floor clean and reducing disease and insects by putting the thinned apples in a bucket not on the ground.

Randy