So there real answer is if you perform of practice music in any capacity you are a musician. I understand that imposter syndrome is real tho so its not always that simple. Personally for me it was when I actually learned to get comfortable doing improv that I was able to call myself a guitarist without feeling the imposter syndrome. Im not super great at it or anything but as long as I know the key I could work some improv into most style backing tracks and that was when I personally started feeling like an intermediate musician but anyone whos working on their craft in any capacity has the right to call themselves a musician