What subjects do you need to be a botanist?

I'm not well qualified to answer this so maybe someone better qualified will come along. Until they do . . .

There are specific degrees in botany. Some schools offer a Bachelor degrees in botany, but some start at the Masters degree level and there are PhD programs in botany. A Bachelor's in Biology would prepare you to go on to a Masters in botany.

The hiarchy is:

Science

. . .Biology

. . . . .Botany

And, there are specialties under botany.