[class.mi] p4 says

A base class specifier that does not contain the keyword virtual specifies a non-virtual base class. A base class specifier that contains the keyword virtual specifies a virtual base class. For each distinct occurrence of a non-virtual base class in the class lattice of the most derived class, the most derived object ([intro.object]) shall contain a corresponding distinct base class subobject of that type. For each distinct base class that is specified virtual, the most derived object shall contain a single base class subobject of that type.

we phrase this rule with the help of class lattice, which means, that what number of non-virtual or virtual base class objects the most derived class object can contain depends on what the class lattice of the most derived class is. We lack a formal definition for how a class lattice of a class form. It could arguably say all rules that depend on the class lattice are all unclear. More details about claiming the definition of class lattice are in https://stackoverflow.com/questions/16886788/c11-class-lattice-with-mixed-virtual-and-non-virtual-bases, this issue has existed since the c++11 standard.