The amount of XP per game month depends on your player level
Each level can add 400 XP.
A level 20 player can add 400 X 20 = 8000XP
This is the limit you see in the barn
Each horse you add XP to can have 800 per level
A level 3 horse can add 3 X 800 = 2400XP
The percent of the barn mean how many XP points get added with each click.
20% will add 20 points.
A barn at 100% will add 100 points and never fail to add.