Would've checked it on the internet, but couldn't find it. I do not play the game so I have no other source, but here to find out.
Hordes are encountered randomly by running through the tall grass, or they can be forcefully initiated using Honey or Sweet Scent.
Source: Experience and Here.
Hope I helped :)
Not that often but if you have a Pokemon with the move sweet sent every time you use it in grass outside of battle, you are going to encounter a horde guaranteed.
I hope this helped!!!