It is perfectly acceptable to create a multi-model report that contains only one model, but in the majority of cases, you will want to add additional models.
Each model in the report belongs to a separate "section", which can be accessed via the tabs at the bottom of the Report Editor window (see Report Properties).
You can only add models that are currently open in the workspace to the report. However, once a model has been added, you can edit the report even if the source model has been closed.