I need to individually and continuosly control 30+ ADCs and DACs simultaneously.
As far as I understand it multiplexing can only use one port at a time?
How do I achieve what I am trying to do?
Are these SPI or I2C parts?
With SPI, connect SCK-MOSI-MISO to them all. Use a Uno or other 328P based board with shift registers to control chip select to one part at a time, or with good coding can read one and write one at the same time.
Can also use multi-channel parts, such as 8-input ADCs and 4 output DACs to reduce # of chips needed.
Example for 8-channel ADC:
http://www.digikey.com/product-search/en/integrated-circuits-ics/data-acquisition-analog-to-digital-converters-adc/2556291?k=adc&pv667=60&FV=fffc0096%2Cfff40027%2Cfff80183%2C3cc004e&mnonly=0&newproducts=0&ColumnSort=0&page=1&stock=1&quantity=0&ptm=0&fid=0&pageSize=25
Can also use a 1284P-based board with 32 IO - tho using 2 pins for serial, 3 for SPI signals, leaves 27 IO for chip selects.
With I2C, connect SCL/SDA to all of them. Select parts that have settable address bits so that multiple of the same part can be used. Or, will need some kind of switching to select between parts.
Example with 2 address bits:
http://www.digikey.com/product-search/en/integrated-circuits-ics/data-acquisition-analog-to-digital-converters-adc/2556291?k=adc&pv667=4&FV=fffc0096%2Cfff40027%2Cfff80183%2C3cc0021&mnonly=0&newproducts=0&ColumnSort=0&page=1&stock=1&quantity=0&ptm=0&fid=0&pageSize=25
I need to individually and continuosly control 30+ ADCs and DACs simultaneously.
As far as I understand it multiplexing can only use one port at a time?
You are quite correct, you can only use one port at a time with multiplexing, that is the whole point.
How do I achieve what I am trying to do?
Get realistic with your specification. A single core processor can only do one thing at a time. Normally you change the converter so fast it looks like they are all reading at once.
If you truly need the conversions to be simultaneous then you are screwed.
Sadly Crossroad's answers are all mostly giberish to me 
Maybe I should be more specific. I need to have 30+ digital feedback loops for a complex power supply.
The intention is simply to maintain a stable DC value on the supplies using the feedback loops.
Some of them will be required to respond to close to 1hz and others in the Mhz.
If I can't simultaneously control these DACs/ADCs then I'm guessing I'll have to do really fast switching. what am I looking at in order to have enough resolution and speed in order to accomplish this?
I think you need 30 simpler analog feedback/control circuits.
Yeah well that's not gunna happen. How do I determine what and how much hardware I need for this? Maybe I need an MPU?
Why not just use 30 ATtiny processors?
Hmmm, this is a good point. I don't like the 8bit resolution though. I want to get as close to a "perfect" voltage source as I can which means super transient response and voltage stability.
I don't like the 8bit resolution though
For what?
You still have your external 30+ ADCs and DACs, it is just one pair per processor.
Don't I need a high resolution PWM to drive high resolution DACs?
Is PWM going to load data into a DAC? Not likely.
Generally you have a parallel interface and a chip select and clock line, or it is loaded serially via I2C or SPI, or some custom serial interface that needs special handling.
16 bit DAC with low pass filtering will yield good results, 14 bit or 12 bit probably not too bad as you are dealing wth power supply tracking and not audio. All 3 need 2 bytes of data, so go 16 if have 16 bit ADCs to monitor the supply voltage. Can always drop the lower bit or two if the signal is too noisy.
Well actually the power supply I'm most interested in perfecting is the one I'm using for audio purposes feeding vacuum tubes, so I want it to be able to handle the audio level transients as well as possible. I read that an analog feedback loop will handle transients far better than a digital feedback loop, so now I'm back at square one, I don't know if I should use digital feedback or digitally controlled analog feedback.
Please explain further.
You have 120V going into a big transformer, you get out some high voltages and a small voltage for tube heaters.
Some of them go thru a rectifier tube, some go thru diodes.
For example, this guitar amplifier,
http://www.ax84.com/hioctane/AX84_Hi-Octane_101004.pdf
Where is there something that can be manipulated/controlled, and how would you propose controlling it?
I'm interested as I made one of these, and have parts to build another higher powered amp sometime with bigger output tubes. (6L6 type I think, or maybe KL88? Some numbers like those, has been a couple years since I did anything in that area).
I also have a Dynaco ST70 (made from a kit by someone before my time, or when I was quite young anyway). It sounds pretty good. Had some hum, I replaced one of the first stage tubes to get rid of that. I bought a set of replacement caps for it and will tweak it up someday.
My plan was to use a series regulator and use the digital feedback to bias the pass transistor.
So for my audio power supply I would want whatever mechanism I use to be able to read and react to transients currents of at least the audio frequency range with the end result being a dead stable voltage supply.
I think you are being swayed by the audiophile gold plated mains connector crowed. You need nothing like that for the highest of Fi circuits. There will be no measurable difference let alone audible difference.
I don't put much stock into the audiophoolery on the internet unless I verify it myself. This is sort of my own agenda.
A sharp transient current would sag the output of the regulator would it not? Surely there must be a potential audible difference during those notes by having a regulated supply with transient response at those frequencies.
A sharp transient current would sag the output of the regulator would it not?
No not with a properly designed convention power supply.