Thank you for responding. Here is a snip from that article:
One item per pack will always be a Mercenary Card and adhere to the rarity guidelines below, and this will always be a Mercenary that you don’t own until you own all Mercenary Cards of that rarity. Once you own all the Mercenary cards of a rarity, you will obtain Mercenary Portraits of that rarity, instead.
Here is another snip:
You will not obtain the same Mercenary Portrait until you own all the Mercenary Portraits for that Mercenary. Once you own all the Portraits for a Mercenary, you can obtain a second copy of a Mercenary Portrait, which will automatically be converted into Coins of that Mercenary: 50 coins for Rare, 75 coins for Epic, and 100 coins for Legendary Mercenaries.
I do not own even close to all rare Mercenary Portraits (and if I do it’s not displaying them as owned). Furthermore, I’ve opened ~50 packs in a row now that are all coins. I didn’t even get one of the 5 missing legendary mercs in those 50 packs. Now, I know the pity timer isn’t published, but I’ve never seen it go higher than 40 packs.