You can't go backwards on the grand line right? So we know he sailed and went through the new world to get a name for himself before he lost his arm to the Sea King so he had to have went around once then. Then he left and sailed off before Luffy set sail a few years later and made it back to MF so we know he sailed around again. So since Shanks has been a captain he's sailed around the world already twice and at least once with Rogers Crew.