Is it reasonable to do number theory without active/good prof in uni
Number theory is perhaps the historically deepest, most studied area of mathematics.  To even begin to understand research in most areas of number theory, you need two full years of graduate coursework over several subfields of mathematics.

What one can learn in an undergraduate course, or even a first graduate course, in number theory is so far from the frontiers of research that it makes absolutely no difference whether the person teaching is active in research or even has ever done research in number theory.  You probably won't get into the 20th century in an undergraduate course, and not much beyond the 1940s in a first graduate course.

Except for a few exceptional students who are taking several graduate courses while still an undergraduate, no one is doing serious, mainstream number theory research as an undergraduate.  Hence, it doesn't matter.  You're not competing with people who have somehow learned lots of number theory already.

I should add that, in the job market for permanent positions in pure mathematics that support significant amounts of research, the sky has fallen.  Only exceptional people are getting permanent research-oriented jobs now.  Given that your job after your PhD (and your postdoc if you get and do one) probably won't involve doing research in either number theory or geometric analysis, you might as well study what you're interested in.