You can do ALMOST anything, but.......
Ever hear the expression TANSTAAFL? (There Ain't No Such Thing As A Free Lunch) Well, unfortunately, it is true in lunch, in life, and yes Virginia, even in MediaMatrix 3.x. While MM is an amazing tool that allows you to do things you never could do before, it too is limited, in that it requires hardware to run on. The more complex the design you make, the more hardware is required. At some point, with a really large system, you can design something so large it will not fit in a single MediaMatrix system.
Fortunately, the software will tell you how much hardware it takes to accomplish a given design in ¼ DSP board increments. Now while you can't buy a fraction of a DSP board, this does give you a way to measure how big your design is and how close to needing another DSP board your design is. You can buy systems that can hold from 1 to 8 DSP boards, most of which can be upgraded with an additional DSP card if needed at a later date.
Each DSP board has the ability to support up to 32 audio inputs and outputs. These inputs and outputs can come in three forms.
First, they can be connected in groups of 8 to Break out Boxes or BoB's. Up to 4 BoB's can be connected to a DSP board, thus providing 32 audio inputs and 32 audio outputs. In addition, each BoB has 8 control inputs and 8 control outputs. Each MM-88xx BoB must be located within 6 feet of the frame holding the DSP DB-9 card(s). Each MM-8802 can be any distance up to 50 feet (15.2 meters) from the frame holding the DSP RJ card(s)
Second, you can get a DSP board equipped with AES-3 digital inputs and outputs. If desired, they can be externally equipped with isolation transformers so they will also meet the EBU standard (with the optional 16XT). Each AES input or output carries 2 audio channels, so there are 16 inputs and 16 outputs which together provide the 32 audio channels. Maximum distance for an AES signal to be run is about 1000 feet.
Third, you can get a DSP board equipped with a CobraNet interface. CobraNet uses standard 100BaseTX (Fast Ethernet) to carry up to 64 audio channels over an inexpensive Cat 5 network cable. Each DSP card connects to a Ethernet switch, and from there to several input boxes called CABs, each of which accepts or sends 8 or 16 line level audio signals (8 Mic/Line inputs available with the CAB-8i, 16 line-level inputs on the CAB-16i). From the switch, you may also connect a virtually unlimited number of audio output boxes (CAB-8o, CAB-16o, or CAB 16d) each of which will output 8 or 16 audio channels. While each DSP board only puts out up to 32 audio channels, you can use multiple CAB output boxes to distribute the signals to many locations. The maximum length of any CAT5 cable is 100 meters (330 feet) without adding another switch to serve as a repeater. Fast Ethernet switches allow transmission of up to 64 channels of audio EACH DIRECTION on a Cat 5 cable or multi-mode fiber between two switches (utilizing full-duplex mode). Gigabit Ethernet switches can take the signals from a large number of Fast Ethernet connections and transmit up to 1000 channels of audio each direction over a multi-mode fiber between switches. For long distance transmission of CobraNet, several companies make single-mode fiber converters. Some of these allow transmission distances in excess of 100,000 meters. Most likely you will need to use the unicast features of CobraNet to gain all the advantages of a large managed switching network to properly route the correct CobraNet audio from Frame-to-CAB or Frame-to-Frame.
Please see Peak Audio's CobraNet site (http://www.peakaudio.com/CobraNet/Background.html) for in-depth CobraNet knowledge and deployment strategies. Also worth mentioning at Peak's site is the CobraCAD software package that allows pre-designing the CobraNet network portion of your overall audio design. CobraCAD allows you to virtually "wire" up your CobraNet and Ethernet network and test this design against the limitations of the cabling, protocols, or equipment that you chose.
If you need more than 32 audio inputs or outputs you must add a second DSP board. Since a MediaMatrix system can have as many as 8 DSP boards, the total number of audio inputs and outputs from a single system can't exceed 256x256.
Each 8 inputs and outputs connects to a single DSP processor chip on a DSP board. Between the 4 processors on a single board there are 255 possible audio paths. Similarly, between the DSP boards in a single system there are 255 possible audio paths carried on the Digital Audio Buss or DAB. In some large systems it is possible to run out of DAB channels.
As you can see, there are 3 basic limitations to MediaMatrix:
You can design a system so large it will not fit in the available amount of DSP.
You can design a system that needs more than 256 inputs or outputs.
You can design a system that needs more than 255 internal DAB channels.
Fortunately, in each case you will know before the hardware gets ordered. In some cases there are special techniques which can allow you to fit complex systems within the above limitations.
If you design something so complex that it can't fit in a single MediaMatrix system, then you can always buy a second system. The two systems can be linked using either AES or CobraNet to carry the digital audio between them.