This is just a small thing. In CK2 you often see armies 5, 7, 8 men being raised. At least in England, twenty-men commanded by a vintenar was the smallest military unit, I don't think any county would bother sending less than that. So, limiting it to 20 wouldn't have a major impact on anything, but it would make the map bit cleaner when you don't have enemy armies constantly stackwiping armies of dozen men.