How many prefects per house?

[potioncat]

No: HPFGUIDX 91403

If this has been discussed, it was a long time ago, so I'd like to 
ask:  How many prefects does each house have? 

 In the first 4 books, I thought Percy was the only Gryffindor 
prefect.  Then in book 5, we have Ron and Hermione.  I had wondered 
before why there wasn't a boy and girl for each house.  In his 7th 
year, Percy was Head Boy.  Did he also serve as Gryffindor Prefect?  
Did JKR just not mention other prefects because it wasn't important 
to the plot?

I'm asking because from OoP, we would think Hermione and Ron were 
the only Gryffindors prefects.  Does that mean that McGonagall and 
Dumbledore looked at all 5th, 6th and 7th year Gryffindors and chose 
the 2 fifth years? So no one else has a chance until they graduate?  
After all, Percy was prefect for 5th and 6th year.(any maybe 7th)

Could there be prefects from each of the 3 years?  I was wondering 
if Ginny and Ron could be prefects at the same time? 

Any ideas?

Potioncat