How far apart to space trees in a food forest

I am just starting my food forest in Kentucky. I started apple trees from seed two years ago and am ready to transplant them to my forest area.
How far apart should I plant them?

That's a tricky question, with a lot of 'it depends'. If you have raised the apples from seed they will be growing on their own roots rather than grafted, which gives them the potential to be much bigger than grafted trees. You can keep them smaller by pruning and other methods or give them lots of space.

Another thing to think about is that they won't be the same as the parent trees, but new varieties unique to your garden. Some might be great, but some, maybe most, will be disappointing - especially if they have been pollinated by a crab apple. This means that you will almost certainly want to remove some, meaning that you could plant closer and thin to the best ones. If you have two great ones growing too close you can always move one via root cuttings or grafting.

Finally, seed grown trees take longer to start fruiting than grafted ones, so don't give up on them to soon if they don't seem productive!

In the place I am developing I put in a few trees with only 15 feet of spacing.  The plan is to keep them very small (and add more) because I just don't eat a ton of fruit but I like variety!