I just purchase a iPhone 12 from T mobile. Do I get a free one year of Apple TV ?

Try redeeming to see. Please click the link below to learn how to redeem the free offer. Also, please remember it is only valid for 1 year and will not work if you are already a current subscriber and you must redeem within 90 days of purchase.

Apple TV+ - One Year Offer

You should be able to get it yes. Assuming you don't already subscribe to Apple TV+ nor have redeemed an offer for Apple TV+ previously. Be it a previous free year or the 1 week free trial.