ABC:John 13

Verse 38
Infidels.org calls as a contradiction, "How many times did the cock crow?"

Mark 14:30 And Jesus saith unto him, Verily I say unto thee, That this day, even in this night, before the cock crow twice, thou shalt deny me thrice.

Mark 14:72 And the second time the cock crew. And Peter called to mind the word that Jesus said unto him, Before the cock crow twice, thou shalt deny me thrice. And when he thought thereon, he wept.

Matthew 26:74 Then began he to curse and to swear, saying, I know not the man. And immediately the cock crew. 75 And Peter remembered the word of Jesus, which said unto him, Before the cock crow, thou shalt deny me thrice. And he went out, and wept bitterly.

Luke 22:60 And Peter said, Man, I know not what thou sayest. And immediately, while he yet spake, the cock crew. 61 And the Lord turned, and looked upon Peter. And Peter remembered the word of the Lord, how he had said unto him, Before the cock crow, thou shalt deny me thrice.

John 13:38 Jesus answered him, Wilt thou lay down thy life for my sake? Verily, verily, I say unto thee, The cock shall not crow, still thou hast denied me thrice.

John 18:27 Peter then denied again: and immediately the cock crew.

The simple answer is twice, but only once after Jesus said this. The rooster had already crowed once, and Jesus predicted Peter would deny him three times before it crowed a second time. Therefore Peter did so before it crowed once more, a second time. There's nothing contradictory about these passages in the slightest.