Not sure of your definition of "not many", but I know several whites who did and NONE of the blacks I know marched with him, then again, most of the blacks I know either weren't in this country or were too young to have marched with MLKjr.
4 white college students were killed for fighting for civil rights in the south. HUNDREDS of white college students came by bus and van to the south to march for civil rights. These students, for the most part, stayed with black families. It was an eye opening experience for them and the families they stayed with.
I know a couple of white families from the DC area that marched on Washington with MLKJr. My friends were just kids at the time, marching with their parents.
Now I'm pretty darn sure that I don't know EVERYONE who marched with MLKJr, therefore it makes sense to me that MANY whites marched with him.