Ask Athanasiou, Gelspan, Flannery, Leggett, or one of the writers of books on the subject written for the casual reader what they think the carrying capacity of the planet is currently thought to be. Ask Houghton, Burroughs, Lovejoy, or one of the more scholarly authors the same question. You'll get lots of hemming and hawing as they say from many of them and others will of course want you to specify consumption rates for non-renewables, renewables and energy but in the end none of them will tell you 9 could probably be sustained at anything like what we consider comfortable and reasonable today.
Either scientists need to come up with radical new ways of growing food, transporting it timely and economically as well as making huge leaps in efficiency and conservation for heating, cooling, transportation etc. or else there simply are not the resources for 9 B people to live a lifestyle even close to what you probably do now. Mostly those authors assume that the inequality will simply become much worse and the numbers of people living a subsistence living will dramatically increase. They just accept that poverty will increase drastically in many parts of the world but in the US we'll just buy smaller hybrid SUV's and more of the poor will have to skip having A/C in their home or apartment. They lament that fact but they accept it as realistic as well.
The IPCC has free info on the subject and no one has more or better research than they do. It is well worth the time to wade through their data if you're really interested in finding out. Working Group II has relevant data that might interest you but mostly you'll have to read the data and put it together yourself. It isn't a popular postion to take and they don't come right out with it. No one can say with certainty but your opinion is definitely in the minority. Best guess is 4 - 6 with a comfortable lifestyle and this is predicated on some serious improvements in efficiency and conservation. If that sounds too low just remember that many credible scientists feel it's more like 2 without a significant portion living lives of deprivation. 8.5 - 9.5 is just too many. Since we're at 6.5 now getting to 4 - 6 in 50 years would be quite a challenge. I've read the justifications for 9 - 10 being the upper limit and found it weak. Admittedly I'm not a practising expert in this field, just very well read, so I don't claim to have a definitive answer. I'm going with the very well supported consensus. We currently carry 6.5 with amazing unfair levels of inequality, poverty, and deprivation along with the rapid and unsustainable depletion of resources. To imagine our population growing significantly without drastically increasing the number or people living lives of horrific poverty is totally unrealistic.
It pains me to take the same position as so many racist xenophobes. That is the reason most scientist prefer to remain quiet on this issue. They are uncomfortable having their research used by such factions and there is not currently much in the way of respectable outlets for such conclusions. Right now population control equates with reducing the numbers of little brown people in almost everyone's mind. It's time to get past that and discuss the issue in a realistic non-racist way. That will probably happen right after we all have nuklear powered flying cars and robot maids.
John Thornton