Paul claimed to be an apostle and saw Jesus. The choice of Matthias to be the successor of Judas by the Eleven was before the descent of the holy spirit at Pentecost. Was Paul the successor of Judas, remembering there were to be but twelve Apostles, and that Matthias was not afterward heard of?

The eleven apostles were acting without authority in selecting Matthias to be the successor of Judas. Jesus had instructed them to stay in Jerusalem and to wait for the outpouring of the holy spirit at Pentecost. It was during this waiting period and before they received the anointing of the holy spirit they mistakenly cast lots and chose Matthias. The Lord did not reprove them for this undesigned meddling with His arrangement. God simply ignored their choice. In His own time, the Lord brought forward the Apostle Paul. God declared to Ananias “But the Lord said to him (Ananias), ‘Go, for he (Paul) is a chosen instrument of Mine, to bear My name before the Gentiles and kings and the sons of Israel'” Acts 9:15. 

The Lord arranged that there should be exactly twelve Apostles. In evidence of this, note how the Lord particularly chose those twelve.  We point to the symbolical picture of the glorified church in Revelation 21:14. In the picture, there are twelve precious foundations of the city, and in them were written the names of the “twelve Apostles of the Lamb . . .”–no more, no less.

Additional Resource:
Christian Questions Podcast
Episode #1113: “Paul or Matthias: Who Was the Real 12th Apostle? Contradictions Series”
Clear evidence to prove Paul or Matthias as the 12th Apostle
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CQ Rewind Show Notes