How fast can you monitor with multiplexing?

So if multiplexing is just choosing 1 output at a time then how fast could I monitor voltage sources using a multichannel ADC?

As far as I understand it, multichannel ADCs use multiplexers so you can only use one of the channels at a time. I'm assuming I would have to constantly switch between them if I wanted to simultanously monitor 4 channels but what are the limitations?

Oh wait I realized multiplexing is just 1 option between i2c shift registers and other stuff.
Which method would be best for simultaneous monitoring of 4 ADCs that monitor 50 microsecond changes in voltage?

"50 microsecond changes in voltage"
That doesn't make sense as a measurement.
There are multichannel ADCs that do simultaneous capture and conversions, you are limited to reading one value out at a time.
You can get fast single and multi-channel SPI ADCs that can return a converted value in 2 or 3 SPI.transfers by commanding each channel one at a time. Different speeds are possible, some can run at 8 MHz SPI clock (2-3uS for a conversion) some at 4 MHz (4-6uS for a conversion) some at 2 MHz (8-12uS for a conversion).

Then your sketch has to put the 2 bytes returned (from a 10, 12 or 16 bit conversion) into an int and do something with it.

By 50 microsecond changes I mean I want all 4 ADCs to be able to read and send simultaneous readings voltage changes at 20khz.
I'm assuming sending 4 different readings into 1 output is going to require some sort of "skipping" of readings at some time frame since you can only send 1 reading at a time.

You can get fast single and multi-channel SPI ADCs that can return a converted value in 2 or 3 SPI.transfers by commanding each channel one at a time. Different speeds are possible, some can run at 8 MHz SPI clock (2-3uS for a conversion) some at 4 MHz (4-6uS for a conversion) some at 2 MHz (8-12uS for a conversion).

Okay since I haven't had a lot of time to learn about digital giberish until now I'm going to assume you mean that the voltage reading from the ADC takes 2-3 signal passes to convert a value for the the uC and with an 8 MHz SPI clock it takes 2-3uS per conversion so It would take 12 uS to read all 4 channels.

Is that right? Sorry

I think once you understand the "digital giberish" you will find a part like this can do what you are after.

CrossRoads:
I think once you understand the "digital giberish" you will find a part like this can do what you are after.
http://www.digikey.com/product-detail/en/texas-instruments/ADS7841E%2F2K5/296-25900-1-ND/2254776

Thanks for the recommendation, it does seem to fit my requirements assuming the data transmission speed is fast enough.
Was my interpretation of your statement correct?