I've seen the phrase 1ターン1度 "once per turn" used in the Yu-Gi-Oh! Card game, so it seems like ターン is a legit counter. But I'm not sure how to pronounce it. From what I've googled so far, some people pronounce it like いったーん which is odd because ターン is a gairaigo.
So how do you pronounce 1ターン?