Last time, I opened 61 packs to get 1 legendary merc without any legendary portrait came meanwhile even.
This time, I opened another 44 packs and there was not a legendary merc still, but there was a legendary portrait around the 14th pack or so. However, according to the official post, “You are guaranteed one new Mercenary Card in each Mercenaries Pack until you have all Mercenaries of a rarity tier”.
I properly quote the official post as an evidence in my topic, as well as their drop rate, and some similar complaints from others. Just too lazy to do the whole thing again: