Overton indexes more than 3M documents from more than a thousand different policy sources, making it much larger than similar systems.
But it's important to note that a "policy source" in this context is a website or domain from which we are collecting documents.
Usually a website includes documents from just one organization, but this varies from country to country.
For example, in the UK a single policy source - gov.uk - hosts documents from all of the government's departments as well as many government agencies.
Conversely in Australia each government department hosts its own documents, so each one is tracked as a separate policy source.
Looking at coverage in different countries
You can still use the number of sources in each country as a very broad indicator of how good coverage is, but if you're focused on a specific geographical area then the best approach is probably to log in and use the Sources page to see directly what is and isn't included, and to use country or region filters on the Policy page to get a count for matching documents.
If you're looking more broadly then useful indicators might be the number of policy documents indexed (if you're logged in you can see the current total on the Policy page) potentially in conjunction with the proportion of policy documents that have at least one reference or citation (approx 30%).
Comparing coverage to other systems
The points above hold true when comparing Overton's index to other systems. You may also want to check subject by subject: the largest policy sources typically cover health and economics and any differences between systems will be more pronounced in areas primarily covered by smaller sources in the "long tail" like education, social services, defense or environmental protection.
Because Overton captures more of these sources it does particularly well when looking at journals or institutions that produce more social sciences outputs.