Hmmm...well that's true. It could be argued that Paul was actually the twelfth apostle. But then he also was martyred. In fact, the term "apostle" gets a bit murky as Paul refers to Apollos as an apostle.
If we're just sticking to the twelve apostles that were actually with our Lord Jesus during His earthly ministry, that would rule out both Matthias and Paul. If not then the issue becomes more cloudy.
I believe that tradition has always held that Matthias was also martyred, though there is a great deal of disagreement regarding when, how and where.
Mistakenly argued since Matthias was chosen to join the other eleven before Paul's conversion.
"And they appointed two, Joseph, called Barsabas, who was surnamed Justus, and Matthias. And praying, they said: Thou, Lord, who knowest the hearts of all men, shew whether of these two thou hast chosen, To take the place of this ministry and apostleship, from which Judas hath by transgression fallen, that he might go to his own place. And they gave them lots, and the lot fell upon Matthias, and he was numbered with the eleven apostles." Acts 1:23-26