The number is Strikers that Laharl can have is equal to his level.

If Papyrus's bone magic let's him make bones, that's what he can do. 'Any' size and shape is a bit much. Let's say that the bones have to be classically bone-shaped and can't be any bigger than trees or smaller than normal bones. Of course, since Papyrus is such an innocent and well-meaning soul, it's not like he'd push the limits of bonemaking anyway.