We usually work (in the city) with 7. On a dry night, a running play that ends up in the middle of the field, or in the side zone opposite a team's bench, we'll use the same ball. If it's in the middle, the U will set up the ball off the wing's spots (or occasionally the B, if it's a long play; he'll then cover the ball until the U gets there). A play the ends in the side zone nearest a team's bench, the short wing will spot the ball on his foot; the deep wing will rotate a ball in to the U, and the H or L will then toss the ball on their foot off once the U has taken the spot.
On a long pass that falls incomplete, the short wing takes a new ball from the ball boy, while the deep wings run it down and rotate it off. (Unless it's a long incomplete pass out of bounds on the offense's side of the field, in which case I'm not worrying about it - if they want a clean ball after the next play, someone needs to get it back to the ball boy.)
On a rainy night, or a night with a wet/muddy/dewy field, we'll rotate a ball in as much as we can, even from the fare side of the field, at the U's judgment.
Out in the country where we work 5 man, we can usually only rotate a ball in if it ends in the side zone of the team on offense. A long pass incomplete in the middle or far side means we've got to chase it down any way, and without 3 downfield guys to get the ball across the field...
Not sure I'm explaining it well...