I believe on average there is a legendary card in 1 out of every 25 packs. Now there are already 10 legendary cards in the Beta edition set, and these new Summoners will add 5 more, so that means approximately 1 in 3 legendary cards would be one of these new Summoners. So that means there should be one new legendary summoner for approximately every 75 booster packs purchased.
I should mention that just because you purchased 75 or more Beta edition booster packs does not mean you are guaranteed to get an airdropped summoner. Some people will have bought a bunch of packs and not get any and someone who bought only one pack may get one. That's just the luck of the draw, and since we are using unpredictable public data from the blockchain to generate the random results it will be provably fair.