What limits the number of PhD students in universities with and without PhD tution fee?
What limits the number of PhD students in universities without PhD tution fee?


I'm not sure countries exist without a PhD tuition fee. Rather there is a scholarship system in place to cover the cost of tuition. The limiting factor here is the number of scholarships allocated by the government. So the answer is the government.


  And when there is normal tution fee, isn't there any limitation for the number of PhD students as long as there are good candidates?


This would be dependant upon the institution. There may be a hard limit imposed by the faculty. Otherwise there is a soft limit in the number of students one professor can reasonably manage.