one by one

This means that "one at a time" each disciple asked him.

Surely not I?

Possible meanings are 1) this was a question for which the disciples expected the answer to be no or 2) this was a rhetorical question that did not require a response. AT: "Surely I am not the one who will betray you!" (See: rc://en/ta/man/translate/figs-rquestion and rc://en/ta/man/translate/figs-explicit)