There are 4 thins to upgrade. You can upgrade everything 20 times. This means to max out everyhting you need 80 skill points. But normal way you can get just 50. There should be another way to get those. Maybe in Hover Crates there could be from 1 to 2. But it would be nice to be able to upgrade everything to the max without having to pay tank coins.
Ya 50 upgrades is pretty good, if you want your tank fully upgraded you should have to pay some money. Plus the people who already have a fully upgraded tank would get nothing.
