3000 were baptized

 

 

A rough idea of what the Pool of Siloam looked like as pilgrims to Jerusalem used it for tevilah, the ritual immersion to purify themselves before entering the temple from the south end, as they traveled along the main road for pilgrims going to the temple. 

This pool was fed by the Gihon Spring, and so never ran dry. It was used for more "official" immersions, whereas the Pool of Bethesda, at the north end of the temple, was more of a "hospital" pool, fed by rain and groundwater, a little dirtier, where Jesus healed the lame man waiting for the stirring of the waters (perhaps an intermittent spring, or ... an angel).

There were hundreds of smaller mikva'ot ("baptistries", the place of immersion) around the temple mount, some public, some private, for the purpose of tevilah (the immersion itself), plenty to handle 3000 baptisms in one day.

For the repetitive ritual immersions needed for handling sacred things and entering the temple, to remove ritual impurity, one need only immerse himself.

For entering a new covenant, such as a Gentile converting to Judaism, one needed to let someone immerse him (or at least witness the self-immersion). This conversion served as a new birth from Gentile to a new covenant relationship with God as a Jew.

John the Baptist took the baptism out of the temple environment, to the wilderness of the Jordan River, and he changed it from an immersion for ritual purity to an immersion for the forgiveness of sins.

Peter took it from John's baptism to the baptism of Jesus, not only for the forgiveness of sins, but also for a new birth into a new covenant relationship with God under the authority of Jesus, with the gift of the indwelling of the holy spirit.