Some quick Wiki research indicates that he knew only of the Baptism of John. (initially)
Acts 18:24-26 NIV
Meanwhile a Jew named Apollos, a native of Alexandria, came to Ephesus.
He was a learned man, with a thorough knowledge of the Scriptures.
25 He had been instructed in the way of the Lord,
and he spoke with great fervor[a] and taught about Jesus accurately,
though he knew only the baptism of John.
26 He began to speak boldly in the synagogue.
When Priscilla and Aquila heard him, they invited him to their home
and explained to him the way of God more adequately.
--- COMPARE ---
Acts 19:1-7 NIV
While Apollos was at Corinth, Paul took the road through the interior
and arrived at Ephesus. There he found some disciples
2 and asked them, “Did you receive the Holy Spirit when[a] you believed?”
They answered, “No, we have not even heard that there is a Holy Spirit.”
3 So Paul asked, “Then what baptism did you receive?”
“John’s baptism,” they replied.
4 Paul said, “John’s baptism was a baptism of repentance.
He told the people to believe in the one coming after him, that is, in Jesus.”
5 On hearing this, they were baptized in the name of the Lord Jesus.
6 When Paul placed his hands on them, the Holy Spirit came on them,
and they spoke in tongues[b] and prophesied.
7 There were about twelve men in all.
[
Acts 18:24-26 NIV
Meanwhile a Jew named Apollos, a native of Alexandria, came to Ephesus.
He was a learned man, with a thorough knowledge of the Scriptures.
25 He had been instructed in the way of the Lord,
and he spoke with great fervor[a] and taught about Jesus accurately,
though he knew only the baptism of John.
26 He began to speak boldly in the synagogue.
When Priscilla and Aquila heard him, they invited him to their home
and explained to him the way of God more adequately.
--- COMPARE ---
Acts 19:1-7 NIV
While Apollos was at Corinth, Paul took the road through the interior
and arrived at Ephesus. There he found some disciples
2 and asked them, “Did you receive the Holy Spirit when[a] you believed?”
They answered, “No, we have not even heard that there is a Holy Spirit.”
3 So Paul asked, “Then what baptism did you receive?”
“John’s baptism,” they replied.
4 Paul said, “John’s baptism was a baptism of repentance.
He told the people to believe in the one coming after him, that is, in Jesus.”
5 On hearing this, they were baptized in the name of the Lord Jesus.
6 When Paul placed his hands on them, the Holy Spirit came on them,
and they spoke in tongues[b] and prophesied.
7 There were about twelve men in all.
[