There's no way to tell how many Burnt Orange on Burnt Orange Challengers were built. According to Galen's numbers there were approximately 457 base Challenger built to US specs made with the interior. (Base Challengers include all the 340 cars too).

According to the Color and Trim Selector this interior was recommended with only 5 colors. Burnt Orange, Go Mango, Black, White, and Beige.

So probably not incredibly rare (I've seen a few) but not incredibly common either. I've seen the interior on all the colors mentioned except Beige, though I think I've seen Beige with a Burnt Orange vinyl interior.