It depends on the situation. If I'm in the mood, I'd take on 5 pieces (excluding sight reading), but that's a bit too much for me atm.
At the very least, requirements for school:
exercises - Hanon, scale, arpeggio
sight read - Czerny, short piece
1 piece by Bach
1 sonatina movement
and 1 piece for my minor instrument (guitar)
I like the idea of building a repertoire, but I can't decide whether to keep reviewing all pieces or just sight read easy pieces and memorize hard ones. Teacher seems to breeze through everything so quickly, I don't get to polish anything unless I give up the routine.