Enterprise architect ER diagrams. Why is the multiplicity value not displayed?

You might have the option Suppress All Connector Labels checked, which would suppress the multiplicities. You can easily test that by giving your relationship a name. If that name is not displayed either it's pretty sure it's because of this setting.

Try unchecking this option by right clicking on the background of the diagram and choosing the option Properties

This is what it looks like after unchecking this option: