It’s theorized you get one roll per difficulty, but killing a higher difficulty before a lower one gives you the lower one’s roll. So if you kill Fyrakk on Heroic, you get three rolls if you haven’t killed him on Normal and Looking For Raid yet.
The way it works is that if you kill it on heroic, it then will roll heroic, normal and LFR, a d if you get the BLP token then it means you didn’t get it on any difficulty. The higher the difficulty the more chances you have, and it seems like mythic might be a guaranteed drop, yet if you’ve killed it on heroic for the week then that’s that. You won’t get any tokens if you kill it on normal or LFR
You can only try for it once a week, so technically each week has a higher probability the more you try, but of course when it’s seemingly set at like a 1% chance and it feels like it only increases the chance by that same amount each week. All in all the legendary has been a failure, in that it’s such a potent time now that it presents clear gaps in damage outputs between players that do and do not have it.
Blizz probably sitting there thinking players just need to get better considering much harder unnerfed versions of Tindral/Fryakk were killed months ago with less gear and no legendary.