Page 34 of the Core rulebook under
Grapple Modifier:
In addition to the modifier to grapple checks, a larger character can grapple more opponents of a smaller size: double the number of opponents the character can grapple at once per size category the attacker is larger than the defenders. So a Medium attacker can grapple one Medium opponent, two Small opponents (one under each arm, for example), four Tiny opponents, and so forth.
I'd say a single attack to grapple them all, although they'd have to be adjacent. The classic example is the giant sweeping up a number of smaller attackers in one fist and squeezing. A Large opponent could similarly use one hand each to grab the two medium-sized opponents in front of him. One attack roll and one grapple check to be checked against the opponent's individual rolls.