We use Michelin maps when we travel in Italy and find them very good. We too have noticed that some roads on the maps don't have numbers, but when we find the roads and drive on them, we never see any numbers. I don't know if they have numbers.
This past March, we got turned around, I'm not a great navigator even with a map and compass, and there was an "un-numbered" road on the map that led to where we wanted to go so we took it. Turns out it was a "strada bianca" or gravel road. It was a beautiful drive, our car made it fine, and we got to the main road just as shown on the map. Since then, we now recognize it as a road that is often included in photos of Tuscany. It is near Pienza and Montechiello. I recite this both as an endorsement of the Michelin map because it correctly showed the gravel road and as an example that sometimes going a little astray can be fun.
By the way, we also ALWAYS take a compass with us.