A lot of that will depend on your insurance policy and maybe the local laws in your area. Since I started the process, ive been on 4 different insurance policies. Only one of them didn't cover me for trans related care. However there are ways arounf that too. Your therapist could bill your sessions under a secondary diagnosis so you don't have issues. Only my old endocrinologist refused to yse an alternative code from gender dysphporia which led to me having to pay for some visits out of pocket. Anyway, insurance policies are becoming more inclusive. And here in NY it's mandated that insurance companies cover certain trans related healthcare costs, so things are getting better. So I would look at your individual policy and find out what they cover and don't cover. If there is an issue, you can always ask you therapist about using an alternative diagnosis when billing the insurance.
I should also note that you want to be aware of you deductible and other aspects of your insurance. For example, I had coverage for bloodwork but got a bill over 1000 dollars because of my deductible. And my hormones cost almost 100 dollars every 1.5 months even with insurance. So be careful and plan ahead for all the potential costs.