Might not be in physics or mathematics but in Life Science it certainly almost is. At the institute I am working right now has about 20 PhD students per full pr

Might not be in physics or mathematics but in Life Science it certainly almost is. At the institute I am working right now has about 20 PhD students per full professor (every 3 years anew, so a Professor has 10 times that number in 30 years). Its all a question of associates and other types of group leaders and obviously delegation.