What is the relation between multiplexing and the sampling frequency in a data acqusition system?
If you want to capture a sinewave of 2kHz then sample as fast as you are able and at the very least you'll need to sample at 5kHz (to avoid unwielding brick-wall filtering on the front end of your ADC).

If you don't sample at a high enough rate you'll get aliasing so, try and sample as fast as you can - that's one channel. Aliasing: -



For ten channels feeding the ADC via a multiplexer then you need to sample ten times as fast as you were for one channel.