The use of UML diagrams to specify a system is a well-known practice among software engineers. UML Sequence Diagrams (SDs) are commonly used to specify scenarios describing message exchanging among objects. A multiplicity is an important modeling attribute that can be used to define cardinalities between classes in class diagrams. The UML Metamodel, which is contained within the UML specification Version 1.5, specifies that SDs can be constructed at two levels of abstraction: the instance level (i.e., using objects or concrete instances of classes) and the classifier level (i.e., using calsses or classifiers). The UML specification states that SDs at classifier level can be constrained with multiplicities but is not prescribed how to denote the multiplicities neither how to interpret them (i.e., its semantics). In this paper we propose a notation to specify multiplicites in SDs at classifier level of abstraction as well as an interpretation based on a UML Metamodel extension. This extension uses inline expressions which are constructs provided by a similar notation called Message Sequence Charts.