How many sq feet of diffusion do I need?

It's 9 x 10 ft. and not meters, right?
First question is, how does the room sound to you now...is it a "live room," meaning if you clap your hands, is the sound reflected like an echo? I can see that your walls are probably reflecting sound.
Secondly, what are you hearing or not hearing from your rig; ie; is bass too boomy or not tight? Are highs harsh or do they sound natural? You can first play with speaker position and toe-in.

In summary, how does your system sound to you right now?

Thank you Lowrider57. Yes, my room measurements are in feet. To be
honest, I think my system sounds very good but frankly I think I dont know
any better because I never had any room treatments. I had a company
come in and do some analysis of my room and they identified various
issues such as modes. Their total solution involves bass traps, absorption
on side walls and ceilings, and diffusion on the front wall. I didnt want to
commit to the entire package, preferring to take small incremental steps.

To answer you question, yes its a live room. I "think" the highs
are bit too harsh. And I have messed around quite a bit with my speaker
position. To me they sound best at their current position.

I think you mean they found some Nodes. I would do the same as you, do it in incremental steps. First of all, you need absorption on the side walls (the point of first reflection where sound from spkrs bounces off).
On the right side, definitely on the door and maybe a strip on the wall next to door. The left wall maybe 4x4' or 2x4 vertical. BTW, you actually need some reflection in all rooms.

Also, something on the REAR wall behind listening position will absorb standing waves. Room treatments such as these will make such an improvement; after that I would work on whether or not you need bass traps. AND I think you should shop around for the right kind of panel; foam would be much cheaper than the one you posted.

Gotta tell ya, you have a very nice system.

IMHO
Start with the bass issues first.
It's counterintuitive but fixing bass nodes significantly improves your rooms mid and high range performance.
Tube traps make a significant difference in the mids and high performance of your room. Once you eliminate the bass nodes, a lot of issues with highs get resolved.

I got a huge improvement in all aspects of my room when I installed a 16 x 48 ASC tube trap in each corner.

If you notice any type of boominess at all, I would start there first, then add diffusers later if you feel it's still warranted