Your “simple question” is a strawman.
Jesus appointed Paul as the 12th apostle as he lay face down on the Damascus road. He never gave the other apostles permission to appoint a replacement apostle.
Without that permission, their actions were invalid.
guess you don’t read the bible much....or just make it up as you go along...Matthias was the 12th apostle....long before Saul started on the road to Damascus...
Acts tells us that Matthias was numbered with the eleven apostles. Twelve is the number that is associated with God’s government in scripture and here we have an interesting parallel between the 12 apostles and the 12 tribes of Israel.
The list of tribes in Revelation 7 adds the tribes of Joseph and Levi and leaves out the tribes Ephraim and Dan. Is it possible that both Matthias and Paul were apostles? Two of them would be left out in Revelation 14:21.