Eduardo is right. You have to actually buy them from the Autoshow (not auctions, not wheelspins, not gifts, not festival rewards). It can be the same cheap car many times to get you to that number. On the accolade itself, you should be able to see your progress.
I bought a bunch of 20K cars during the Secret Santa. I singlehandedly got everyone the Santa Suit and the Peel and gave a few hundred people a reasonably-priced car.
Not looking forward to having to do this, but that is the last accolade I need in that group… just takes too long to buy the cars one at a time… will be a multi month process I guess. And then having to gift them all and waste more in game time that I could be racing!
I would skip the gift step, I’m not even sure totally new to Forza players would much appreciate it. I mean it’s a nice idea, and if you took the time to paint and tune them nicely then maybe it would benefit both you and the recipient but otherwise it’s probably just more work for whoever gets them to collect them, regift them, auction them, or delete them. Please save the whole “gift” mechanic for cars worth it. Just my opinion of course.
Another idea, imo a better time spent one, I’ve been slowly working through it doing a few at a time while waiting for other stuff. With a car that has some skill mastery benefits like super wheelspins, xp, cr, and fp. Buy it cheap, fill the minimum rewarding skill mastery tree, then remove from garage. And whatever you do, don’t be one of those who gifts a car that has the good skill mastery bits already used up.