It tends to rise as you progress through levels. The higher level, the faster the car, the more stress on the parts. 5 seasons ago I would use a maximum of 5 or 6 parts per race, per car. Now I'm up at 11 or 12 at circuits like monza. You have to develop manufacturing up to a point it can keep up as you progress. Hope this helps
AFAIK parts usage is linked to the development of the Design Facility. As you develop and increase the level of your design HQ you will find the quantity of parts you require to repair your cars after the race also increases.
If you want to keep in top of this you will need to develop Design and Manufacturing as a pair and ensure Manufacturing is never more than one level behind Design HQ.
You can also increase your stock by 5 parts per day by watching adverts if necessary.
Ok, thanks a lot for the advice