- if baking 60x potatoes in the oven, will they take longer than the time if there were just a few ?

- and if I cooked them in advance, would they warm/re-crisp if I then put them all in for half an hour or so on the day

These points are discussed on the potato/taco bar for 100 page, it takes 60-80 pounds of potatoes to do that. NEVER good reheated- see the cook and hold method, which is good for about 6 hours.

Yes they take longer, but not much, it is NOT like a microwave where every potato increases the cooking time a lot. Preheat the oven thoroughly, at least 20 minutes.