Android Go is designed for phones with limited resources and comes with a variety of apps, including Maps Go. The latter offers limited functionality compared to the main version, but doesn’t require a lot of resources to run. The fact that it has just passed 500 million downloads on the Play Store is a good indication of the success of the platform.
The app can technically be installed on non-Go phones, but it’s so limited in functionality compared to the main Maps app that it doesn’t make sense to install it on a regular phone, especially since it doesn’t provide navigation. (this is done through a separate app).
So that’s a good indication of how many Go phones have been sold. In fact, even though the figures aren’t accurate, since Maps Go is pre-installed on Go phones and it’s not such a useful or regular handset, it’s easy to estimate that around 500 million Android Go devices have been sold.