1,000 cars cap in the garage!! YAY!! ...but seriously, most people who'll play when the game will be dead will only have access to 500-600 cars due to so much stuff being locked behind Festival Chorelist.
I’m 97% sure that the locked content will go into the Autoshow before the game retires.
They've done it for FM7, but as far as I recall they haven't done so with FH3. There's about 40-50 cars in FH3 that were Forzathon/Championship rewards that are not available anymore since that game reached EOL. That is one major glaring issue with the way they've handled how they give out new cars; it's fine and dandy for the most part when you play the game in any current situation (especially since cars return every once in a while so fi you missed one the first time around you still get a chance), but someone picking up the title on the cheap in say, 3-4 years will miss out on a lot of content.
Pretty much all cars locked behind events can be found in the AH though. It's nothing more than an Autoshow with over-inflated prices.
IIRC some cars, like the Porsche 928 in FH3 were offered only once, so there's only a finite number of them available. If no one that owns one puts it on the AH, no one will get one.