Biggest question to ask is: what is your budget?
Many
amateur and intermediary engineers will record their mix live either directly from the board (matrix or auxiliaries) or split their signals to a dedicated
console and processing straight to ADAT/CD/Mini-Disc. This is how it's been done for years and many still do it that way.
With advent of the computer and its portability, more and more engineers are no longer recording with linear-systems (mix to source) and instead using DAW (digital audio work stations) + hard drive systems.
The DAW method is great for live because it cuts down on gear (computer + interface vs.
console + processing for almost every
channel) and you don't need dedicated engineer/tech for it. With a DAW setup, you can get away with as little as laptop and a small rack with your interfaces and some optional simple analog processing (
comp/
gate/limiters) for the overly-dynamic audio sources (kick, snare, poorly mixed video feed,
etc) that won't
play nicely with your interface's preamp. The processing isn't always needed, but if you're recording a live event and a sources clips your preamp, there's no going back. Ever hear that saying "garbage in garbage out?"
Also, with a DAW, you can mix and remix your recording to your heart's content. Mixing through a DAW gives you a much wider range of processing and effects options for practically little to no cost because it's software (ie: cheaper to produce software
effect than to produce a physical
unit, ship it,
stock it, sell it,
etc).
I'm not totally sure what you're asking, so I'll keep it short and sweet. Add in details in your reply.
Capturing live audio?
A few room mics to capture room ambience and audience. Ideally, you'll want a
snake splitter and feed all the sources in their native state to your DAW rig's interfaces/preamps. If you're short on inputs/processing, you can have the
FOH console feed you the grouped-sources (drums
in one feed, guitars
in one feed,
bass in one feed,
etc) via subgroups/auxiliaries/matrices/direct-sends. If the
FOH live rig is not capturing or mixing their audio great for whatever the reason, you can always bypass it and feed your DAW rig with your own mics. This can get enormously complicated and can create a lot of clutter.
Capturing video?
Take a feed from the video source. Be sure to have a
direct box as well as wide variety of adapters and signal converters on
hand to be prepared for whatever type of audio feed the video crew will give you. Despite all the advancements in digital broadcasting standards and video, there are still some video crews out there that'll feed you unbalanced signals in form of
RCA, 1/4", 1/8",
etc. It's almost 2010 and some in the video world are still discovering balanced audio. There are do-it-all kits/interfaces you can buy that are specifically designed for pro-audio professionals to accept audio feeds from video folks.
Basic schematic of a DAW recording rig:
[Computer] ---data cable--- [Audio Interface/Preamp] ---xlr/trs--- [Mic/Source]
You'll want a computer with a modern
processor such as a core 2 duo, core i7, or xeon running in the 2ghz range or better. Because of your 21-24 inputs you'll likely be using plethora effects and you'll want to avoid
latency issues for mixing/recording, go ahead and stuff the computer with as much ram as it can hold. You'll want ample hard drive storage and then some to back it up. If you're going the laptop route, run your external drives via eSATA.
While I love protools, they're extremely proprietary as a DAW
system. Their entry-level/intermediate LE-line is expensive and won't satisfy your input needs. With 21-24 inputs, you'll need HD-line protools hardware and software. The HD-line gear is the among the best and is terribly expensive. I can't justify that being cost effective for live recording
system unless you plan on commercially distributing and selling your recordings. An protools HD rig is a serious studio beast that is useful for more advanced live setups (think Rolling Stones).
With that, I'd recommend you consider solutions from one of the core audio-interface manufacturers such as MOTU, PreSonus, Focusrite, Apogee,
etc. Apogees stuff is amazing and works beautifully with the
Mac + Logic-Pro combo, but it's pricey. I tend to prefer MOTU and PreSonus because they deliver the best bang for the buck. Also, the MOTU and PreSonus gear easily daisy chains to handle larger volume of inputs.
The bare components of my personal rig consists of the following:
-late-2007 model Macbook Pro w/ 2.6ghz Core 2 Duo + 4GB ram
-MOTU 896HD 8-channel 24-bit/192khz Firewire Audio Interface
-Apple's Logic Studio (Logic Pro, Mainstage, Soundtrack Pro,
etc)
-Firewire cable
If I were in your position and needed to record 21-24 inputs, I'd
pick up two more MOTU 896 interfaces and
daisy chain them. This would give the ability to record a total of 24 inputs. Since I'm mostly a studio guy, I don't know about
latency issues with recording 24 simultaneous tracks. I usually record no more than 6-8 tracks at a time and I have no problem. However, when I'm mixing down (after I've recorded) 20-30+ tracks with full effects and procesing, my laptop shows it. I'll have to freeze tracks to free up
system resources and keep
latency to a minimum. You may just benefit from a more powerful desktop if you're going to regularly be recording and mixing down 21-24 live sources with lots of effects because powerful laptops are pricey.
As far as software, use whichever works the best for you. They all work well. Just make sure it's compatible with your interface. Most use something along the lines of "Core Audio" drivers. Some DAW software do things better than others, but it ultimately comes down to personal preference. I use and learned Logic Pro because I was able to receive it for steep discount. I've heard great things about or have had good experiences with the following MOTU's Digital Performer, Reason, Cakewalk, CuBase, Neundo, Sony Acid/Soundforge, Ableton Live,
etc