There is actually some interesting stuff behind thirteen.
In the original twelve tribes of Israel, the tribe of Levi was set apart as priests (Numbers 1:49-50). This included the partition of land. The Levites received no land for themselves, they were supported by tithes on the other tribes.
So certain counts have eleven tribes (because Levi is set apart). Sometimes, there are twelve (not counting Levi).
This is because Joseph took a wife from Egypt, and had two sons: Manasseh and Ephraim (Genesis 46:20). These are sometimes referred to as "half-tribes". Sometimes the two half-tribes are counted with the other ten.
So sometimes, there are 11, sometimes 12, sometimes 13.
We see a similar occurrence with the apostles. There were twelve apostles, minus Judas. Judas was replaced by Matthias. But, Paul was also a replacement. So, sometimes 11, sometimes 12, sometimes 13.