I regularly record 16 channels on my 3 year old laptop and never have any problems recording or playing them back. Where you DO start to need computer horsepower is if you try to add real time effects to those tracks--but there are ways around that.
To answer the original question, you need some form of interface to convert your 16 channels of audio to data and feed this into your laptop--and this is where you are going to struggle. With a laptop you're limited to USB, Firewire or maybe Cardbus interfaces--and I don't know of any of these set up to handle 16 analogue channels. Most are set up to take some analogue but receive other sources in some digital format such as ADAT. As an example, I use a digital mixer to feed up to 32 channels of ADAT via an appropriate interface into my laptop. There are ways around this if you really need 16 channels--but you'll probably spend more than the cost of your laptop on an awkward solution.
I think you have to ask yourself if you really need all 16 channels at once in a home studio. Not many home spaces I know of can really handle this sort of recording--a pro studio would have all sorts of acoustic baffles, vocal booths, etc etc. Although I can DO 16+ tracks at once, I rarely use this facility except for the occasional recording of a live event--not at home.
If you can re-think things and live with 8 channels at a time, then the world opens up to you with all sorts of interfaces to choose from and a choice of USB or Firewire. Especially if you're only getting started in home recording, I'd suggest you start a bit simpler and play with layering tracks for a while to learn what you actually want and need.
Bob
