On console this week (at least, up until tomorrow when the week resets), the featured Event kingdom is Divinion Fields. It’s understandable that you’ve received a larger amount of Herdmaster because it’s a blue ultra-rare and the event chests apparently prefer to give out the ultra-rares associated with that kingdom.
For Divinion Fields, these are the cards featured in the “Mess with the Bull” event chest:
Herdmaster (ultra-rare)
Atlanta (city epic from questline)
Ragnagord (epic)
Bul’Tauros (featured epic, purchasable with Glory)
Orion (legendary)
Just looking over this list, I can make some rough predictions about what you can expect from 5 event keys:
3+ Herdmasters OR
1 or 2 blue ultra-rares from another city AND
1 Bul’Tauros… but only if you’ve already purchased him with Glory!
Your chances of getting Orion are really, really, really low 
But if you do get a legendary from the chest, it will be Orion rather than someone else.
I am basing my preliminary observations above from my recent event chest pulls from the PC/Mobile version, which last week featured Sword’s Edge. Out of 44 or 45 event keys used, I received x25 Knight Coronets (blue ultra rare from Sword’s Edge). I was rather upset that I was getting troops from Adana, Khaziel, Stormheim, Whitehelm and Zaejin rather than the Shadow Dragon legendary.