You quote I Corinthians 15:5 “…and that He was seen by Cephas, then by the twelve.” Your next comment following says “(later Paul makes the twelfth).” As I read Acts 1:22–26 the twelfth was Matthias not Paul.
In the book The Troubled Church (page 162) I state what most conservative scholars accept — Paul counts as the twelfth apostle to replace Judas, not Matthias.
You’re correct, the first chapter of Acts records the remaining group casting lots to pick a replacement for Judas. But what instruction had they been given? Jesus “commanded them that they should not depart from Jerusalem, but wait for the promise of the Father”.
But Peter (acting when waiting was called for) took it upon himself to motivate the group to pick a replacement, to which they agreed; they didn’t follow Jesus’ directions. The next error came from limiting God’s actions by only providing two choices, and asking the Lord to pick one. The question wasn’t should we pick a replacement for Judas, it was which of these two will you pick Lord?
Most commentators note something along the following — Matthias may have been a good guy, but he’s never heard from again, yet Paul wrote a majority of the new testament, possessing apostolic authority while doing so. In short, man chose Matthias, God chose Paul.
At the least, you have to add Paul to the twelve (making the apostle group thirteen), but it’s generally viewed Paul as the twelfth apostle, and not counting Matthias as the group didn’t follow God’s instructions.
In case this thirteen-counted-as-twelve sounds a bit odd, it’s not the only time it happens in the Bible. Ask yourself, how many tribes of Israel exist?
If you think 12, you’re off by 1.