Okay, maybe millions is an exaggeration but when millipedes decide to invade a home they can do it in a big way.
Millipedes tend to stay in the ground, require a lot of moisture and are nocturnal, therefore, to reduce a millipede problem minimize excessive moisture in the yard.
By keeping the grass short and edged, bagging clippings, removing fallen leaves and other debris as soon as possible, watering the lawn in the morning if needed and reducing mulch to not more than three inches you can minimize a millipede invasion.