For a lawn that is in good shape, two passes with a rolling lawn aerator are enough. For a lawn that has large areas without any grass, four passes are ideal to ensure that grass seeds have plenty of surface area to grow.
The number of passes necessary to properly aerate a lawn depend on the current grass growth. Multiple passes ensure that the lawn has plenty of soil plugs and holes. After seeding, grass begins to grow and root in the holes and around the dirt soil plugs. The keys to effective grass germination are proper aeration, good-quality seeds and sufficient water.