Prefect system

[Darla]

No: HPFGUIDX 46171

Hello!
   Have just been looking over the poll questions etc, and something 
struck me as odd. One of the questions was 'which boy do you think 
will be made prefect?' and i just realised it doesn't make sense. I 
mean, only one boy and one girl can be made head boy/girl, but the 
system i've always known is that quite a few people can be made 
prefects in the same house etc, they just need to have a good 
reputation. (remember mrs weasley telling the twins she'd noticed 
that neither of them had been made prefects?) But a lot of people 
talk about prefects as if only one boy/girl can be a prefect from 
each house. help?
  Darla.