Generally the overhead compartments (sometimes only one side of the plane) can accommodate 3 bags with wheels; anyone who places their bag in such a way that other bags won't also fit is a "hog".
Most FA's, when they see bags that are poorly place will place them to MAXIMIZE available storage.
There are an annoying number of aircraft where, despite having a carry on that is entirely legal sized, doesn't fit the 'logical' way around in the overhead bin (and yet will on your connecting flight with the same airline!) They really should give a maximum size for the aircraft type, smaller on those planes with limited sized overheads.