I submitted the question for the July #ForzaMonthly:
"Forza 7's IndyCar Bounty Hunter marked the end of 20 months of new cars and event engagement; will Forza Horizon 4's content support go on for that long - or longer?"
Matt Pickering from Playground Games answered:
"Yeah, I think so. I know our future schedule, which I mentioned, goes way into the future. And where it ends, I don't think that's the end. Wherever there's appetite we're going to keep making content.
So yeah, for the foreseeable future we're going to keep support going for Horizon 4."
https://youtu.be/4VFJFP6h43Q?t=3082That doesn't specifically answer the Stories count question or any other specific content info, but that's where the '20 months' comes from.