In fact, it's not a question of maximum mapping segments.
I generated gmapsupp.img from Topo US 24 K installed in MapSource.
A strange thing occurs : if I use patched MapSource with the view of generating gmapsupp.img on my computer hard drive (and not directly on a microSD) before copying it on a SD, map is not displayed on my Montana when I put microSD inside.
In fact, if I want the map to be seen on my Montana, I must generate gmapsupp.img directly on a microSD. I can then copy this file on another microSD and it will also work.
Any explanation ?