You can certainly save yourself time by prepping the potatoes ahead of time, and leaving refrigerated in water. Add 1 tablespoon of citric acid (concentrated lemon juice or white wine vinegar) to each gallon of water. To save further time, you may wish to add a step and parboil the potatoes, then roast prior to serving. If the venue does not have an oven you should be able to find or rent a suitable one to transport to finish on site. This will ensure the crispest end result. Otherwise the potatoes may not have the desired crunch if roasted prior and held too long. As for quantities, it’s always best to err on the long side. Idaho potatoes are always the least costly ingredient and it is not uncommon for guests to indulge once they see a delicious roasted potato as an offering – actually saving you some money as they satisfy their appetites on the side potato dish, and less on the proteins offered.

Here's a link to the proper steps to storing peeled potatoes, which should be helpful: https://idahopotato.com/dr-potato/proper-steps-to-storing-peeled-potatoes