How much compaction should I get from crushed rock?
Don't put down a zillion inches of stone, then expect it to compact nicely anyway. You simply won't get as good of a result, as solid compaction as you will by laying down a couple of inches, compacting, then more stone. And you will spend less time compacting each layer anyway. 

If you do it in layers, it will be clear how much you need to lay down to get your desired depth.