How many Prefects are there?

[Robert Jones]

No: HPFGUIDX 84808

Nothing in the plot turns on this question, but I would still like 
to know the answer: how many Prefects are there?  I mean, is being 
appointed a Prefect a one-year gig, or do those who are appointed 
Prefects in their fifth year remain Prefects in their sixth and 
seventh years?  In other words, at any given time, are there *two* 
Gryffindor Prefects or *six*?  Maybe someone who knows the British 
boarding school system can shed some light on this.