how much does gravel compact?

I think you should shoot for 4" to 6" of compacted crushed stone. Don't worry about how much it will compact, use it and adjust it's thickness to set your subgrade.

Do use crushed stone (also called ABC for aggregate base course) which has all the different sizes of stone and fines. It compacts the best and all the angular pieces lock together for a firm surface. Keep the material slightly wet for best compaction. Don't use a stone all of one size. It doesn't compact well and has a tendency to roll.

Paul.

There is not much airspace in base rock. It dosen't compact much. 5%-10% I would guess (not having a verifiable figure). 4" to 6" is a 33% range, shouldn't be hard to hit that mark. Order enough for 6" and don't worry if you get 5.5".

How much you need is mainly dertermined by the undisturbed soil benieth. If you live on sandy or swampy ground, you need a little more base but if it is hard packed clay, maybe you need a little less.

Compact the soil under the base too, especially if it has been disturbed (added not removed)

It varies depending on the gravel. As noted there are some types better than others.