There is no soundcard with 16-24 mic pre's like that....there's the Aardvark Q10 with 8 mic pre's but you would have to have 3 to get 24 and I don't think Aardvark is multi-driver capable...
You should get external pre's as it's quieter and a better way to do it.
Opus

Opus is right again, dammit! :D
The upside of that route is that as computers and converters evolve you still have nice outboard pres to interface with your consistantly upgraded computer peripherals...

But which is more cost effective: to buy saperate units of pres or a multichannel mixer? Can u recommend the brand of the necessary hardware?

There are ways (expensive ways) that will get you that many channels. I personally can get 16 at once with only one sound card...

You will need to but external preamp units with digital outs. I am using the PreSonus DigiMax, which has 8 mic pres. I then ADAT out of that puppy to the computer. There are lots of computer sound card that support ADAT input, and I know for a fact that the Echo cards support mulitple instances.

Here is one way to do it (this is my method of getting 16 channels, btw):

Buy 2 PreSonus Digimax's and once Echo Layla. From the first Digimax, ADAT lightpipe 8 channels into the Layla. (8 channels, so far) Now, from the second Digimax, take the 8 analog outputs, and wire them into the 8 analog inputs on the Layla. (Now we have 16 channels)

Once again, Echo supports more than one of their cards at once, or you could find another card that is compatible that that accepts ADAT inputs, and get 8 more channels...

Dawman1234

In my case, I'm trying to figure out what to get for two mics, maybe a few more, but not 24. Maybe the following will help. I am not endorsing any of it, just listing it to give you some more ideas. Scenario #2, for example, gets fairly involved.

I will apologize ahead of time if all of the following creates more confusion for you than providing help.

This site organizes the various audio interfaces into categories based on what the PC interface is: http://www.audiomidi.com/hardware/audio.cfm. There are definitely a lot.

Scenario #1: similar to Opus's Q10 idea but with the MOTU 896

(http://www.motu.com/english/motuaudio/896/body.html). It has 8 mic preamps You would need three of these. What is appealing about this is that it is firewire attached. You would have to find out if 3 could be supported. On the negative side, I have heard that MOTO and INTEL/AMD don't always work very well. You would have to do some investigation to find someone who is using 3, with the same motherboard/chipset you have, and see how satisfied they are. To get started, search here, at http://www.unicornation.com/ and http://www.audioforums.com/forums_frame.html.

Scenario #2: similar to Doublehelix's scenario but with RME and Presonus

The RME Digiface (RME Reference PC) will give you 3 ADAT I/O interfaces on your PC meaning 24 channels.

Then use 3 of the Presonus DigiMaxs (http://www.presonus.com/html/products/digimax_96k.html). Use its ADAT I/O to connect to the RME.

The above gets the mic signal into the PC. You still need some way to listen to the sound.

You could use the RME S/PDIF Out and connect that to a Lucid DA9624 Stereo 96k D/A and then connect the Lucid's analog out to your powered monitors or amplifier.

Problem: Word Clock

With the above, you now have 3 pieces of digital equipment which need to have their digital clocks synchronized. The RME and the DigiMax both have word clock I/O BNC connectors. I have not learned why the Lucid D/A does not (the Lucid A/D does). To synchronize these things you would then need to get something like the Aardvark AardSyncII ((deadlink)).

Scenario #3: Panasonic WZ-AD96M instead of the Presonus

At the http://www.da7.com bulletin board I read about the Panasonic WZ-AD96M (http://www.panasonic.com/proaudio/96ser.asp) which is like the DigiMax in that it has 8 mic preamps and analog to digital converters.

Scenario #4: multichannel mixer

You mentioned this, instead of separate mic preamps. In this case I believe, based on all the reading that I have done, that mic preamps in a mixer are not as good as those in an separate mic preamp. There are analog mixers and digital mixers.

The trick here is to find a mixer with 24 mic preamps and that also has 24 outputs that you can send to the 24 tracks in your PC. Mackie, for example makes a 24 and a 32 channel analog mixer (http://www.mackie.com/Products/8Bus.asp). This page contains the PDF files for the mixer's manual: http://www.mackie.com/TechSupport/Tech_Library/Owners_Manuals/8Bus_OM.asp

Each of its inputs has what is called a direct out. If I understand everything, each of those direct outs could be sent to an analog-to-digital converter and then into the PC. So, in this case you would need to connect something like the Mackie 24-channel analog mixer to 3 of the Panasonic Ramsa WZ-AD96 (similar to the WZ-AD96M execept it does not contain preamps) and then connect them via ADAT to the RME.

Problem: Latency

I don't totally understand this problem but latency has to do with the amount of time it takes for a signal to make its way from the mic, through the preamps and analog to digital converters into your PC and onto the hard disk and then back out of the PC through digital to analog converters to your speakers. Again, I don't have any first hand experience with this, but to exaggerate, I think one COULD setup their system really badly so that if you said "hello", it would come out your monitor speakers a half second later. This can obviously be very annoying when you are recording your initial 24 tracks and be very bad when you are re-recording one of the tracks while listening to the other 23 tracks.

So you need to find a PC audio interface that minimizes this problem. To get a better idea of what this all about, search on latency, here and at other websites you may be familiar with and the unicornation and audioforums site that I mentioned above. RME talks about it briefly at RME Reference PC, scroll down near the bottom of the page and click on "Zero Latency Monitoring (ZLM®): Automatic monitoring of the input signal with zero latency". MOTU talks about it at http://www.motu.com/english/motuaudio/896/cuemixplus.html.

In summary, because you need so many inputs at the same time, I think that complicates your configuration which means you have to do a lot of research and ask questions before you actually buy anything.

Again I apologize if I have created more confusion. As I was learning about all this I reached a point where I THOUGHT I was comfortable and then I discovered the word clock topic (the Aardvark text above). After researching that topic I said, OK, now i can figure out what equipment to get. No ... I then read about latency, so I had to research that and see how that affected what I would buy.