Fortunately for us, there have been apostles beyond the original twelve.Have you even bothered to read Acts 1?
It's not an assumption that Paul isn't of the Twelve. It's a matter of fact. The one who filled Judas's position was Matthias. "He was numbered with the eleven apostles" is pretty explicit...
Barnabas comes to mind. (Acts 14:14)
Even Jesus is called an apostle in Heb. 3:1.
Who knows how many there have been over the last 2000 years.
Praise be to God.