I'm guessing 100-200 people. Just thinking through the logistics. We will be at a church with large ovens, so I will bake them while setting up tables. I have a roaster I can keep filled at the counter, and maybe large coolers for storing more.
And they'll be trickling in over 3 hours, so it's not like there is going to be one big line that needs to move quickly.
DH is a chef and has done a lot of catering. He says you really don't want to do this and agrees wholeheartedly with low-key.
I also used to work for a few different caterers. I would NOT rely on the church ovens to be working properly, evenly, and up to full temperature and cooking the potatoes THERE at the church. That is soooo asking for trouble. What happens if you find the ovens won't heat evenly? Or if you turn up the temp to 500 degrees but they are barely getting the potatoes warm? Or taking way too long to heat up, let alone cook? You will have partially baked potatoes or raw potatoes in the middle. You also don't want them FINALLY done by the third hour.
My suggestion, and what I've seen the catering companies do is pre-cook 3/4 of the way or almost fully cooked and use the ovens on-site to just heat up the potatoes or to
finish cooking. This way if the on-site ovens aren't working properly or taking too long, you've taken the ONE factor that could cause the biggest problem and party disaster out of the equation. Pre-testing the church ovens with 2-3 potatoes is not the same as having an oven filled with 40 potatoes, on 2 racks, and trying to heat them all at once.
I would also wrap them individually in foil. Who knows what was cooked in those ovens the day before and a stinky residue is still in there, imparting flavors to the neutral potatoes. And people can individually pick up a potato without dirtying the others. There is always one guest, and you have teens, who will dig through to find the biggest or smallest potato, touching them all to get to it.

OR while going for a second potato, the accidentally drop the one already on their plate into the batch, and it was already covered with toppings.
If you do decide to leave the potatoes uncut, make sure you have a place and utensils where people can cut them before they get to the toppings.
YES! Go to the dollar store and buy several paring knives and leave them just before or in front of the toppings bar so they can cut into their potatoes. And buy a several extra serving utensils. Some are put into a different toppings and get gunked up.

You'll get teens who use the first serving spoon he picks up, keeps it to use in
every topping he scoops into.
For toppings, make sure you have some protein toppings. And have a couple dairy-free substitutes like sour cream, for those with those issues. Maybe also, a fat-free sour cream.
Anyway, a baked potato bar sounds yummy!

I'd be up for it.

Your DD & her BFF made a good choice if you can pull it off without major hitches.