It should definitely be Motorsport with the largest car lists in the series as it would be a compilation of the franchise, and given the focus on more realistic driving, you'll definitely want to add more cars to simulate driving those cars in a more realistic fashion. Horizon's physics are more arcade-like, but the selling point of the Horizon games is the open world alone. The selling point of Motorsport is track racing and being able to simulate any and all kinds of cars from everyday cars to off-roaders, supercars, race cars and more.
Motorsport 4 was said to be the best Motorsport in the series mainly because of its large and diverse amount of cars, not to mention its career mode and online features.
You want a horrible game like Motorsport 5 where they cut down on cars and get rid of fan favorites and some of the unique vehicles? And plus, no matter how focused or strange the cars may be, it would seem fun to drive them all in racetracks (real and fictional) and to experience them with more realistic physics. Given the physics engine of FH5 now supports four-wheel steering on some cars, imagine what the next Motorsport's physics engine could do, and that they could simulate cars with a complex physics system (from the Mercedes Benz G 63 6x6 AMG to three-wheelers like the Peel P50 and Morgan 3-Wheeler).
What's to be disappointed with realistically expecting over 1,000+ vehicles? We've had at least 786 cars in Motorsport 7 (in my honest opinion, Motorsport 7 still lacked some extra cars), and given what was added to FH4 and now in FH5, it could easily exceed 1,000+ with no problems, and not to mention, considering they're restoring older Xbox 360-era vehicles and adding new vehicles behind closed doors.
In fact, every car that has been in the franchise since Motorsport 3 are considered to be premium in quality (as they are fully-modeled inside and out), it's just a matter of what was left out in the Xbox One era that needs restoring. Those left out from the first and second Motorsport were only to remain as standard vehicles.
Believe me, if the next Forza Motorsport has below 700 cars, let alone 1,000+, given what we have so far in the Xbox One era, believe me, it would be an absolute disaster of a game that we've been waiting for, for years in the making, given we haven't had a new Motorsport for four years and counting. Horizon can have 1,000+ cars too, but for Motorsport, it is absolutely mandatory and necessary for it to have the largest car list the franchise has ever seen.
In GT Sport and GT7, this is perfectly understandable given that in the previous Gran Turismo titles, there were PS2-quality cars that seemed all but possible to convert all to premium just right away, and that they're the vast majority of cars as opposed to premiums, and they're doing it in a slow process.