I have a 2017 jeep cherokee with 23k miles. Because of a damaged tire not repairable, should I buy tire 2 new tires same axle mounted on the back... or do I need to buy 4 tires because of the 4 wheel drive (usually in Auto Mode)?

I'd purchase 2 new tires and put them on the front. Use the used tire as your spare.

I would contact the dealer for assistance or read the owners manual to see the factory recommendation.

On a 4wheel drive vehicle, the only thing that matters is that all tires are the SAME SIZE, as different sized wheels and tires can ruin the 4 wheel drive unit as if they are different sizes, then they will turn at different rates.... And that's not good!