I have a two part question regarding to the Peter’s denial of Christ. First, did Jesus tell Peter that he would disown him three times at the Mount of Olives (Mark and Matthew) or before they went to the mount of olives during the last supper (Luke and John)? And the 2nd question is did Peter deny Christ before the rooster crowed twice or once (Matthew, Luke, and John state once Mark states twice)?
There is no contradiction here. MML & J all agree that the scene occurred at the end of the Lord’s Supper scene. Perhaps it happened as they were leaving as a little side conversation. That fits the description. By the way, the Mount of Olives is where the Garden of Gethsemane is. So, when Matthew and Mark say that after they sang, they went to the Mount of Olives, it could just as well have said that after the song they went to the Garden of Gethsemane. It is a little different that M & M say after the song they left for the Mt. of Olives, then describes the three rooster conversation, then has them actually travel to the Mount of Olives. My only thought is that perhaps all got up to go, then Jesus had the little conversation with Peter as they were preparing to leave for the Mount of Olives after the formal Seder service had ended. What is unanimous in all four gospels is that the more private conversation between Jesus and Peter happened at the end of the Seder meal, before they reached the Mt. of Olives.
About the number of times the rooster crowed, surely this cannot possibly be an important question. Who really cares how many times the rooster had crowed? But, since you asked, we should attempt an answer. There are two possible explanations for this one. The more likely one is that the rooster had crowed twice already, but ML & J did not bother to mention the unimportant detail that the crowing had actually already happened twice. The other possibility is that the crowing had only happened once. This is possible because some of the manuscripts of Mark do not have the additional phrase “two times.” It is possible that a scribe, trying to “improve” the gospel of Mark, added the little explanatory “two times” so as to make it clear that this was before the third crowing. In this case, it is possible (but not likely in my opinion) that the scribe who foolishly attempted to improve the text actually introduced an error and that the rooster had, in fact, only crowed once. This scenario requires an assumption of a scribal change for which we really do not have good evidence for and it requires that the number of “crows” was actually only one, for which we also do not have any evidence. This is why I think it is doubtful. In any case, the only possible reason to worry about this one is if we have someone looking for an excuse to say the gospels contradict (which they do not) and finding a ridiculously small detail as a supposed reason to say there is a contradiction. There is not a contradiction, either way, but I am not completely sure which is the correct explanation. I lean toward the conclusion that the rooster had, indeed, crowed twice already, which is implied by the gospel account in general, but that Matthew, Luke and John did not bother to mention that it was the second time the rooster crowed after which Peter completed his denial. As is usually the case, when we find an apparent contradiction it is not in fact a contradiction, but the result that different witnesses point out different details.