If I buy the game (whether digitally or one of those old-fashioned physical copies that I hear some people still get), everyone on my system can use their gamertag to play the game.  So if you have family members/friends/roommates playing it with their gamertags, that's extra players but not extra copies sold.

Not sure it was ever that "huge" as a franchise. Most of the European sales on the 360 must have come from bundling as it was pretty much impossible to buy an Xbox 360 without Forza once Forza 2 released.

That said, as long as it's selling enough to continue being made, I'm happy, Forza Horizon is my current favourite racing franchise. I'm just a bit shocked that having bought an XB1 for Forza Horizon 2, I'm enjoying the 360 original a lot more now that it's been made XB1 compatible. Still can't wait to play FH3 once I'm done with these 2.

You would not believe the number of people I have spoken to raving about Horizon, who thought it was a simulation racer.

I would hope Horizon 3 outsold the last couple of games......especially H2.....considering it has 2/2.5 the install base nearly. It is an amazing game and deserves high sales, especially considering it's probably the best Xbox Exclsuive this gen imo the sales should be bigger and comparable to other "big" series. 
Would have liked to see how Persona 5 (supposedly niche) could have done worldwide at release.....considering in Japan alone it's nearly comparable to H3 in its biggest 2 markets.

The decline probably has something to do with less bundling, it went annual, and digital sales taking from physical numbers. Oh, the genre has also declined in popularity.

So yeah, annual franchise probably selling a few million a year while using a lot of recycled content. Not too shabby.

Ah, I didn't think about that. I also didn't realize there were so many people sharing games, especially this soon after release. Really puts things into perspective, now that I think about it. A game like GTA5 has sold over 60 million copies, but it's entirely probable that it has been played by well over 100 million people -- especially counting used sales.

Should have sold 5m, great games deserves more sales.

The decline in the series is more due to the bundling than anything else. I wouldn't worry if I was Microsoft. They sell 2 million copies of Forza every year.

Although I would prefer if they release the games every three years instead of two.

Edit: I forgot to add the growing digital share. When you compare this gen Forza and last gen you have to take into account that a huge chunk of sales come from the digital store. 

Non bundle copies of Forza games on the 360 are usually between 2 million and 2.8 million. What's the percentage of people who bought the bundles actually wanted Forza, that's something no one here can answer.

The series was never as big as it seemed to begin with.  It was always mass bundled in the past which inflated it's numbers. 

VGC updated its sales.
Forza Motorsport 4 (X360) - 515,859
Forza Motorsport 3 (X360) - 481,324
Forza Horizon 3 (XOne) - 382,089
Forza Motorsport 5 (XOne) - 325,760
Forza Motorsport 2 (X360) - 319,212
Forza Horizon (X360) - 316,820
Forza Horizon 2 (XOne, X360) - 315,749
Forza Motorsport 6 (XOne) - 221,042
best selling Forza game in the last 5 years.