After Peter and John were arrested in Acts 4, the number of believing men grew to about how many?

Answer: 5,000

Many who heard the message believed, and the number of men grew to about five thousand.

“They seized Peter and John, and because it was evening, they put them in custody until the next day. But many who heard the message believed, and the number of men grew to about five thousand.”— Acts 4:3-4 (BSB)