Promo Pack allocations are based on Tickets and Engaged Players.
Stores are notified of their allocation several weeks before the related set comes out. (Promo Packs are sent out for each Standard-legal set.)
Every week, typically on Wednesdays, you will receive an email from the WPN that contains your metrics as of that day.
Keep in mind that the metrics are based on a rolling calendar year. As you record new events, old events that are older than a year will rotate out.
Promo Pack contents vary by season. Typically, they'll include four cards in a combination of rares/mythic rares, foils, and promo-treated cards with special borders, frames, or alternate art.