If your engine stock is below 20, when it's time for a restock, you are only restocked to 20 total engines. e.g., if you have 10 engines in stock, the supplier gives you 10 to bring your total to 20.
You get 20 new engines after 20 races, not at the start of the season. But for the first race of the season you get 2 more new engines automatically installed.
If your engine stock is below 20, when it's time for a restock, you are only restocked to 20 total engines. e.g., if you have 10 engines in stock, the supplier gives you 10 to bring your total to 20.