It depends on your situation.
Our application for ArcGIS Pro consist of more than 10 add-ins (more than 50 buttons and tools) but we use one extension for all of them. One add-in (module) is responsible for extension state managing. Other add-ins (modules) reacts to extension state changes via condition which changes depend on extension state.
To put extension in each add-in makes reason if each add-in is different application.