Yea, just rng, sometimes you can do a whole act without a single herald.other times you’ll get 3 or 4 in a single zone. I just pop and go and not worry about it, you’ll find replacement shards.
3 or 4 in a single zone, that has to be some big zone with several levels then.
I havent had better than 2 heralds with small zones.
My sample is still low so thats that.
Have had Ires in one single unique kill several times so I see how 3/4 is possible but hardly in a place like Burial/Crypt/Mauso.