Peter taught about Jesus and his resurrection from the dead. This troubled the Sadducees because they did not believe in Jesus' resurrection from the dead.
## the number of the men ##
specifically males. Any women and children who were there were not included in the count.
## since it was now evening ##
It was common practice not to question people at night.
## was about five thousand ##
"became about five thousand" or "grew to about five thousand".