9:17-18 What baptism did Paul undergo here?

Paul was baptized both with the Holy Spirit and in water here, and while it is not specifically stated that Paul was baptized with the Holy Spirit, that is implicit in the fact that Ananias laid hands on him solely for the purpose "…that thou mightest receive thy sight, and be filled with the Holy Ghost".

Paul received his sight, which implies that he had also been baptized with the Holy Spirit. He was then baptized in water (CP Ac 22:16).

When used on its own in scripture the word bapitzed always refers to water baptism (CP Mt 3:6, 13-16; Mk 1:4-5, 9; 16:16; Lu 3:7, 12, 21; 7:30; Jn 3:22-23; 4:1-2; Ac 2:38, 41; 8:12-13, 36-38; 10:47-48; 16:15, 33; 18:8; 19:3-5; 22:16; 1Cor 1:13-17).

Conversely, whenever anyone was baptized with the Holy Spirit in scripture it is qualified as such (CP Mt 3:11; Mk 1:8; Lu 3:16; Jn 1:33; Ac 1:4-5; 11:16).

The evidence that one is baptized with the Holy Spirit is that they will talk in tongues (CP Ac 2:1-4, 14-18), and this Paul did more than them all (CP 1Cor 14:18).