Suggested

Number by car according to ranking

I mean that you as the team manager, to take one or two numbers  (from #2 to #99, also not repeating numbers, like in the real F1) for your cars & let the #1 for the Drivers World Champion 

How would permanent numbers work if you changed league and someone in the league you are moving to is already using your numbers? Presumably the devs would have to code an option to change "permanent" numbers and then also update the livery. Sounds like a lot of work when weather, advanced strategy and the transfer market are on going issues nearly two years after the new game was launched. Personally I'd like to see the devs focus on these problems which are potentially causing people to quit the game.

It's easy. IF your nº are already chosen, you can choose two different numbers from the ones that are free

Easy to say, but how easy to code and what won't the devs be working on whilst they're coding this enhancement? Don't forget the original post was about the numbers displayed on the livery not about the actual number that is assigned to your cars.