I agree with everything so far. Two is better than one. Adults or young adults may be a better choice for you. As said, kittens are a real handful and that may be too much to ask of your parents. Also, two bonded adults are so
much harder to find homes for together (I'm assuming you'll be going to the shelter). How many boxes depends on the cats. I've always had one box and two to three cats with no problems. I keep it extremely clean all the time. This involves scopping at least
two times a day, but usually a lot more. Basically every time I walk by. And giving it a full cleaning at least once a week. But I don't use scoopable litter.
As for the expense, increase in daily expenses for two is negligible. You'll buy bigger cans of food and that's much cheaper than the little cans. I get the 12.5oz cans of Wellness which is about 1/2 the cost of the 3oz cans. That feeds my two guys for about 2 1/2 days. The real difference would be in vet bills. Pretty much double although most vets will give a small multi pet discount if you bring them at the same time. Keep in mind, emergencies do
come up! Always have an emergency fund or a designated credit card. I really can't stress that enough! Within the first couple of years Nanook wracked up close to $3000 in vet bills.
Don't want to scare you, but it happens all the time and you need to be prepared.
As for leash training, I used to walk one of mine every day. He loved
it but, as others have said, it's more like a meander than walk.
If you've really thought it through and you're ready for the responsibility, I hope you can talk your parents into it. They bring so much entertainment, joy, fun and love into a home. I can't imagine life without them!
P.S. One more thing about the food, shop around! I know you said you wanted to stick with PetCo (put Walmart out of you mind for pet food!) but when I went to check the price for Wellness at PetCo, it was way higher than my little local pet store. You can usually find better deals on the higher quality foods at smaller, independent pet stores or online.