What is the ruling on someone who swore not to return to several forbidden websites, then returned to them? Does he offer one expiation, or an expiation for each website he returned to?

It is forbidden to break an oath (unlawfully) regarding not entering prohibited websites. If you break it, you have sinned and are obligated to offer expiation. If there was a single oath not to enter such websites, then it is dissolved by entering one of them, and a second expiation is not required for entering another. However, if you swore a separate oath for each website, then breaking each one necessitates a separate expiation.