As many as you can.
You can pressure test the water passages, which will find cracks in cylinders, porous lifter bores, etc.
You can also pressure test the oil passages, which will find a drill-through on an aftermarket block, crack in a main/cam passage, etc.
Most shops that are equipped to do it have the specialized plugs/manifolds to block whichever holes need to be checked. They will put 30-40 lbs of air pressure through and watch for a leak-down over time. If it leaks, then they will start spraying around with a soap/water solution looking for bubbles.