Is it reasonable to do number theory without active/good prof in uni I'm from the top math university in my country (it's not really world class but on average one guy from every other batch in my uni goes to Harvard/MIT/Princeton/UChicago/Stanford/UMichigan for PhD). 

I'm in my second year and I'm broadly interested in number theory and geometric analysis, but there's no "good" faculty in number theory in my university (there's only one prof and he don't do any active research and don't have much reputation outside my country), so I'm almost on my own to study number theory.

It's pretty hard to learn everything by myself with simultaneously handling the coursework, so I'm wondering whether it's a good/sensible idea (wrt say grad school application where I need to compete with many people from top colleges in USA who are taking legit number theory courses by world class number theorists) to switch completely to geometric analysis and ditch number theory ? I think I can focus more on geometric analysis in that way too -- the only thing that is bugging me from switching completely is that I like number theory more than geometric analysis.

There were one guy from my college who went to Princeton for number theory who wrote a solo number theory paper as an undergraduate, but I'm nowhere close to being that smart. Also I can do reading courses/summer internship under some active number theorist researchers in my country, but the problem is that you need to know some nontrivial amount of number theory before you can do some internship under some reknowned number theorist !
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.