Plus there is no Trading Cap for theese Items (I think) => you can get all of them pretty quick.
Incorrect. The limit is 50 per day, and that includes if you've sent other stuff.
However, this might be more of a limitation on how much the receiving person can have waiting to be collected. What I mean is, if you send me 50 items and I haven't yet received them, you can't send me any more, but perhaps if I receive them you could then send more. What I can say for certain is that I had an alternate account send me a bunch and it cut it off at 50.
How do people have so many of these if we can only win it once with the new seasonals? Do u win them in open lobby online races as well?
People can dupe them, for one thing. In theory, you could send me helmets without ever losing any of your helmets by simply restoring a backup save, and I could likewise send you helmets the same way. If you make another account and backup before it receives suits and helmets, you can get unlimited helmets by simply restoring all the time.
However, the random helmets and suits seem to always be of certain series, and seem to basically be the most bland/generic ones. You'll get a lot of Horizons, Choppers, and maybe a couple more types of helmets, as well as a lot of Bi-color, Trainee, and maybe one or two other types of suits.