How often do professors accept new graduate students?
There is no way to guess how many students an unknown professor is likely to accept, because professors vary wildly in the number of graduate students that they accept each year.  Reasons for this variation include, among others:


Amount of funding varies, from professors who are totally broke to professors who need lots of warm bodies to feed into their research machine.
Personal taste and scholarly style vary, from some professors who love to be in charge of a big lab with lots of things happening, all the way to others who would prefer to have just an occasional disciple or two.
Career and life events vary: a professor about to go on sabbatical may not want to take any new students, while one in the fresh excitement of an opening line of research may want many.
Program structure varies, from some programs that admit students almost entirely without reference to professors and the students don't even link up with professors for their first year, all the way to others where the individual professor is almost entirely in charge of admission.


The best way to guess how many students a particular professor is likely to be hiring is to look at the web pages of their students and see when they started their programs.  Even that, however, only gives you some indicators about the past, and not the likely near future, given all the sources of variability.