I got a new line (and #) and did not update it with steam app prior to doing so. Im still logged in and can use my authenticator when it requests it. What can I do to update my number seeing that I don't have access to the phone number attached to my steam account? Its been a while since I got the new # and I have never gotten around to changing it, and I don't know who owns my old #.
You can easily do this by going here if you have access to your phone number, which you don't but, if you start the process, there's a certain step where you can choose something like "I can't confirm the phone number" and you can go on following that path.
I'm not sure about it as I never did it but you will probably have to contact Steam support (good luck with that).
When you added Steam Guard to your account it should have prompted you with a recovery code and hopefully you wrote it down somewhere. This code can be used in order to regain access to your Steam account or to change your phone number.
Anyway, if you still have the Steam Guard app on your phone you can still use it without requiring the recovery code to sign in.
In order to change your phone number you can choose between a SMS to your old number or that app. The app also includes a recovery code mentioned above and you can review it whenever you like, but the generated token should work just fine.
If for some reason you can't use your mobile app AND you don't have your old number (which obviously is the case) you can still remove your old number by verifying your account via E-Mail.
Only if all of those steps fail you'll likely have to contact the support team.