Mizar is a star system that is composed of two pairs of stars, mizar A and mizar B, or four stars in total. What do astronomers call a system that is composed of more than two starts?

Mizar is a quadruple or quaternary star system.

When there is more than two stars, it is called a star system or multiple star system or stellar system.

Depending on the number of stars, they are also called a triple, quadruple or quintuple star system or trinary, ternary, quaternary, quintenary, sextuple, sextenary, septuple or septenary star system.