Canon question re: prefects

[Jen]

No: HPFGUIDX 60779

I can't remember whether it's been introduced in canon, or if there 
is a typical "traditional" selection re: prefects, but do we know if 
there is only one prefect per house?  2 prefects, a boy and a girl, 
per house?

I have seen the discussion threads of how you might (or might not) 
become one, but didn't see (and perhaps I missed) a discussion on the 
number of prefects at Hogwarts.

Thanks for any help you may have!

"Jen"