Trucks usually have many gears because the engine rpm range where it has best efficiency is very short. Therefore, many gears are required for the truck to cover wide moving speed range. F1 cars have 8 gears now days. Most modern road cars have up to 6 gears, some supercars have up to 7 gears. So, stats wise 6 is enough, but some odd balls need more.
Given the variety of mods, there seems to be the need for more than 6 gears to replicate transmission systems. My oppinion is that allowing 8 gears should cover mostly everything, but it's up to Scawen to decide.