Wiki says 1 horse per tick.
I believe stables produce 2 per tick unless it's something else. Was this changed this age, and or is it a % rate now (look below).
"Houses horses and increase their production rate." - This seems to suggest that you can increase the % of how many are produced like i.e. 2%. This is confusing at least for me.
Didn't it used to read houses and produces horses?