Jinshan was able to come because he had many brothers and sisters. With his family's small amount of land, it would not matter if he and his mother did not come.
The other 12 men also had many brothers and sisters in their families. When the time came for the crops to be harvested, it wouldn't matter if they weren't present.
Xue Erfu had only found young and strong men who could make time from their family's work. They had agreed that it would be 15 coins a day, and when the second half of the year came, these men would start harvesting rice for their family.
There were people from their own village and people from other villages, but they were all people who kept their promises, were decent people, and were familiar with Xue Erfu. They would definitely not stand him up.