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 just passed 500 million downloads on the Play Store is a good indication of the platform's success.
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 much sense to install it on a regular phone, especially since it doesn't provide navigation (this is done through a separate app).
This is a therefore good indication as to how many Go phones have been sold. Indeed, even though the figures aren't perfect, since Maps Go is pre-installed on Go phones and is not so useful or regular handsets, it's easy to guess that about 500 million Android Go devices have been sold.