The mark of the beast is 666 which is the mark of man.
Both Hebrew and Greek do not have a separate numbering system so letters of their respective Alphabets have numerical values.John was a Hebrew writing to Hebrews and hiding the information from the Roman overlords.
Some of the very early Greek manuscripts have the number as 616. Regardless of which number is chosen Satan and God are at war with each other and Satan is going down fighting and will take as many people of this world as he can. Satan and his team is going to make it as hard as possible for God to win but Win God does.
The Mark of the Beast is 666
14 Because of the signs he was given power to do on behalf of the first beast, he deceived the inhabitants of the earth. He ordered them to set up an image in honor of the beast who was wounded by the sword and yet lived. 15 He was given power to give breath to the image of the first beast, so that it could speak and cause all who refused to worship the image to be killed. 16 He also forced everyone, small and great, rich and poor, free and slave, to receive a mark on his right hand or on his forehead, 17 so that no one could buy or sell unless he had the mark, which is the name of the beast or the number of his name.
18 This calls for wisdom. If anyone has
insight, let him calculate the number of the beast, for it is man's
number. His number is 666.