I'm wondering how to go about getting a full drum set (up to 8-9 mic signals) into my little Mbox. How do i mix it down to a lower number of racks before putting it into the Mbox?
In order to achieve that with an MBox, or any simple stereo-input type interface, you'll need enough microphone pre-amps and a mixer to mix down all the microphone signals you want to use into a stereo output that you can patch into the LINE level inputs of the MBox.
You could get a small mixer that has built-in microphone pre-amps and use that to feed the MBox. Something like a little Mackie mixer or something similar will be the cheapest solution, although not necessarily the "best sounding" solution.
If you are just doing simple demos for yourself and maybe some friends at home, then something like that would probably be fine.
However, if you are expecting to achieve major label quality sounds at home, then you aren't going to get anywhere close.
Be aware that since you are submixing the drums to a stereo pair, you will have NO CONTROL of the levels of individual drums when it comes to mixing time.... you will be stuck with whatever drum mix you came up with when you recorded. Unless you have a LOT of experience doing this and know how the drums should sound to make them work right with all the other instruments and vocals in the finished mix, you are going to have a tough time getting a sound you are happy with in the end. Beginners to engineering and mixing often making the mistake of listening to each track by itself and making it sound big and full by itself, but when they put everything together into the final mix, they wonder why it all sounds muddy and undefined. That's another lesson, though! 
My personal recommendation for anyone trying to record high quality versions of their songs that they can sell to the public, or use to shop to labels, publishers, etc., is to NOT record at home! You can get away with electronic based music at home, especially if the only "acoustic" source you are recording is vocals (but you'll still need a great microphone and preamp and good recording space even to get a great vocal sound), is to rent out a bigger professional studio for at least your basic tracks.
Unless you are independently wealthy, you simply are not going to be able to set up a properly designed acoustic recording room, and afford all the expensive high-end microphones, pre-amps, and mixing boards that a professional studio has.
I've been doing this professionally for over 20 years now, working for several of those years as a staff recording engineer at a major studio in the Seattle area. I have had a high end project studio in several homes and office locations since the early 90s. Up until 2006, I always rented out bigger studios when I needed to record drums or track large bands all at once. It wasn't until a couple years after we moved into our current large home with some decent sized rooms, and I had saved up a lot of money, that I was finally able to remodel and set up a room with proper acoustic treatment, and buy enough quality microphones and other gear to get a really good drum sound in my own studio. It's still a small room compared to the big studios I'm used to, and I still rent out big studios when I want that big room rock drum sound that I simply can't get in my own studio.
You can always get several songs ready to go, with maybe some scratch parts laid out in your MBox Pro Tools system, and then bring those Pro Tools project files into a big studio that has a higher end Pro Tools system with a great sounding room and enough good microphones, pre-amps, and channels of quality A/D converters to record everything onto separate tracks.
Then, you could bring that project back to your home and work on vocals or guitar overdubs or whatever else you can do at home at a good quality level, and then send the files back to a professional engineer for mixing.
I know that all the "pro" audio dealers want to sell you on the hype that you can buy all this low cost gear and crank out major label quality recordings at home without the expense of a big studio or hiring an engineer who knows what they are doing, but they are only trying to sell you equipment! I'm all for buying some gear and having fun recording yourself at home and trying to learn how to make good sounding recordings, but you need to realize that it takes a LOT of practice and hard work, along with great gear and good sounding rooms, to even get close to that major label sound. The time that you spend trying to get a home studio setup and teaching yourself how to make decent sounding recordings is time that you are taking away from songwriting and practicing your instrument.
You should go to the articles section of this web site, and in the Recording/Production subsection, read the article titled "Can You Really Do It All Yourself?".
Everyone should read that article and think about it before they start spending lots of money on home recording equipment that is never going to give them the results they are after without LOTs of money in upgrades and MANY YEARS of practice and experience!
Anyway... plenty of things for you to think about... more than you asked for, but I didn't want you running out there spending more money for something you might not be happy with without knowing a little more first.
Good luck with it!