Every 50 Chaos Legion packs you buy, you are guaranteed at least 1 promo card. You may get more than 1 as each pack individually has a 2% chance to receive a promo card as well.

I advise you checkout the Leaderboard page then hover over the Doctor Blight image under the Rewards column. If you do this, it will tell you how many promo cards that player is guaranteed to receive depending on how many packs they received.