Need one or two tires? Had a flat

15.5
T6

right sidewall slashed

16k miles

should by 1 new tire? put rears up front and the new and old in the back ?

or just get 2 news for front

One option is to buy a used tire. Same model, specify tread depth you want to match, and it's for the right side of vehicle.

As I understand it, to place the least wear on the transmission and AWD rear differential, you want to have:
FRONT tires match in their circumference (R & L)
REAR tires match in their circumference (R & L)

That's why when you rotate tires on an AWD, you exchange the front and rear wheels, each side keeps the same two wheels.

The other design rule you're respecting is that steel belted radial tires have a specified spin direction. If you move a
wheel from the left side of the car over to the right side, the tire will be spinning in its anti-direction. The grooves in the tire channel water
properly rolling in their spin direction, but will hydroplane if spun in the opposite direction. The spin arrow is molded into the tire's sidewall.

So, your options are:
- Locate the correct used tire
- Order new right and left pair

discount had a great sale and so did Michelin - gave a good credit for the 3 good tires - got 4 new Pilots for under 700, includes roadside too. I just could not bring myself to have Pirelli on front and Michelin on back - besides the scorpion model on the car is no longer available - the difference is more than subtle but not extreme - besides new tire

I've had one tire replaced. SAme make and model. MUST meet the tolerance on the circumference. It's in the Owner's manual. IIRC it turned out to be about 1/8" of the thread wear for 16" wheels, but please don't quote me on this, writing by memory. IIRC I called the dealer and they confirmed.

Tire Rack would shave a tire for you, you have to provide measurements. So do the many tire shops. SUbaru dealer suggested to do it when I had one tire blow on the Legacy.

With AWD - difference between rear and front also matters, it affects wear on the bevel gears. Latest Haldex systems are more tolerant to that though.
HTH, YMMV.