10 axes, 10 swords, and 10 spears would equal 30 provisions.
Using more advanced troops like Light Cavalry, Mounted Archers, etc. use more provisions per unit. So if you have 10 LC, that's 40 provisions.
The amount of free provisions you have is basically the number you have to stay within when building troops and buildings. To increase that number, you need to level up your farm. That gives you more provisions for more troops and more buildings.