There seems to be a lot of confusion about this achievement due to the poor description concerning variations and reverse layouts.
Basically, if it is on the list when going into Solo, it counts. There are more "variations and reverse layouts" in the full list.
For example, all three of the Silverstone tracks listed in the main list count. Each counts as one track.
Unfortunately there is no way to see what you have or haven't driven, but there are more than 50 in the list. I went and drove a few of them that I knew I hadn't raced on, on a quick race weekend.