“So help me God” is optional. Some do, some don’t. 0bama used it on all three occasions. With his hand on Lincoln’s Bible for the first swearing in and then on two Bibles for the second swearing in (Lincoln’s and Rev. M.L. King).
However, looking at google images, I see Lincoln's fat red Bible for 0's 1st Innaugural. But Roberts messed up so they had to redo the Oath in private. There I see 0's hand on a blue-colored book. For his 2nd Innaugural, because the date fell on a Sunday, he was sworn in privately before the public event. Then he messed up that 2nd public Innaugural and I have no idea if they ever did a redo. (They probably didn't because the public 2nd Innaugural was actually a redo.) If they did, that would mean the 0 has taken the Oath five times.
Perhaps this has been researched more thoroughly by others. I am doing a quick, shoot from the hip, on the fly "research".