Jamieson, Fausset and Brown on Matthew 18:21

21. Then came Peter to him, and said, Lord, how oft shall my brother sin against me, and I forgive him?--In the recent dispute, Peter had probably been an object of special envy, and his forwardness in continually answering for all the rest would likely be cast up to him--and if so, probably by Judas--notwithstanding his Master's commendations. And as such insinuations were perhaps made once and again, he wished to know how often and how long he was to stand it.

till seven times?--This being the sacred and complete number, perhaps his meaning was, Is there to be a limit at which the needful forbearance will be full?