Paul is referring to James, the brother of Jesus as an apostle. A word for word translation appears here:
ἕτερον δὲ τῶν ἀποστόλων οὐκ εἶδον εἰ μὴ Ἰάκωβον τὸν ἀδελφὸν τοῦ Κυρίου
Other. moreover (but/also). of the. apostles. none. I saw. if. not. James. the. brother. of the. Lord.
Biblehub (Sanday: Ellicott's Commentary) states: "From the form of this phrase it would appear that James, the Lord’s brother, was considered to be an Apostle." It suggests that if the word 'brother' meant that he was a cousin of our Lord, then he could have been James the son of Alphaeus, one of the original Twelve. If so, it would seem that none of the gospel authors realised that James son of Alphaeus was related to Jesus, given the perfunctory way he is introduced (example: Mark 3:18) and never subsequently mentioned. Alternatively, Sanday says that if James was either the son of Joseph alone or of Joseph and Mary, then Paul must have used the title in the wider sense in which it is applied to Paul and Barnabas. If these are the only two alternatives, then I see this as more likely than that James was the son of Alphaeus. In support of this, the gospels tell us that Jesus had a brother called James and that he was not one of the twelve:
Mark 6:3: Is not this the carpenter, the son of Mary, the brother of James, and Joses, and of Juda, and Simon? and are not his sisters here with us? And they were offended at him.
Returning to Paul, he referred, in 1 Corinthians 15:5-7, to the ‘twelve’, James and then the ‘apostles’:
1 Corinthians 15:5-7: And that he was seen of Cephas, then of the twelve: After that, he was seen of above five hundred brethren at once; of whom the greater part remain unto this present, but some are fallen asleep. After that, he was seen of James; then of all the apostles.
This can only be the same James, 'brother of the Lord'. A straight-forward reading of the passage suggests that Paul saw the twelve and the apostles as a different groups - to him, being one of the twelve was not the same as being an apostle, and vice versa. The juxtaposition in the same sentence of James and all the apostles could once again suggest that, to Paul, James was an apostle.
In summary, Paul almost certainly did regard James as an apostle. Biblehub offers two alternative explanations, one of which speculatively provides Scriptural support for James as the son of Alphaeus and therefore one of the twelve. Paul, himself, does not refer to James as one of the twelve, but seems to regard the twelve as a group distinct from the apostles.