Group GameObjects together in a lightmap with Baked Tags

The two images below shows two views of the same Scene:

In this example, all the GameObjects have the same Baked tag, and Unity stores all lighting data in one atlas. In this example, one GameObject is assigned a different Baked tag, and Unity creates a separate lightmap for it.

Select the CPU or GPU for baking

Troubleshooting baked lightmaps

Unity Manual

Group GameObjects together in a lightmap with Baked Tags