Buying 750 packs during the pre-sale will get you at least 15 (1 BCX) of the exclusive promo card.
After this exclusive promo card and pre-sale, there will be 13 other airdrops that won't stop until a new milestone is achieved when selling out Chaos Legion packs. My guess is each milestone will be 1M more packs sold.
So when each milestone is reached, an airdrop will occur. The airdrop will work very similar to the promo card airdrop, however, the guaranteed amount of packs required will increase quite a bit (I'm guessing around 300-400 first airdrop).
After the first airdrop, the subsequent airdrops will always increase in guaranteed amount required as all packs bought since the pre-sale count towards these airdrops.
So if you buy 750 packs during the pre-sale, those 750 packs will count through all 13 airdrops. That is why buying early in pre-sale is pretty significant as you will get a chance for all airdrops through the public sale.