It might be good even to tell your therapist you are worried she will change her phone number, even if you think it does sound clingy. (I find it helpful to share out these thoughts, and figure my therapist can help me more the more he knows, though I realize that's not the approach for everyone!)

I do think even if she does change her number, you will probably be able to find her new one online - she's running a business so it's in her interest to let people get in touch.

I do use email, and it's been useful for me - I usually tell him I don't need a reply, because I know he gets deluged by mail and keeping up with email correspondence is not one of his strengths! I initially contacted him by email and his email address is on his website so getting it was never an issue.