Multiplicity refers to the number of object class instances of one class may be associated to the other class. An n+ symbol indicates that there can be n or >n associations of the object class near to the n+ symbol to the other associated class. A solid ball indicates 1 or >1 associations and a hollow ball indicates 0 or 1 associations.
An example association is as shown below –
Multiplicity symbol indicates that as many users may be connected to as many workstations.