There’s no physical limit, but the best advice is to keep the number of localities small, and make sure that they are all geographically close. The application will try to keep all of the locations in the notebook visible on a map — so if you add Portland, Maine and Portland, Oregon the scrolling will begin to drive you 
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Multiple localities in a single Notebook work best if the localities are closely related geographically, like Hamtramck and Detroit in Michigan, or the Ute Mountain Tribal area, which spans Colorado and New Mexico.
All of the localities appear in all of the views, including summary tables.
The map viewls place all of the localities in the notebook, on the same map, so if you add Portland, Maine, and Portland, Oregon, you’ll spend a lot of time scrolling!
Putting several unrelated localities in a single notebook increases data-load times as well.
Some use-cases that work:
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Adjacent places that share common characteristics.
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Counties and their county subdivisions or their school districts.
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Upper and Lower legislative districts, when the lower districts nest into the upper districts, as in Oregon
Some use-cases that cause problems:
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Putting several counties into one notebook. This can be useful for comparisons provided you’re willing to allow the view to load all the data
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Geographically distant localities.
See Shortcuts for managing the "Active Notebook" for tips on managing the Active notebook.