Depends...
What are you limitations?
-max box size?
-available power?
-what intended frequency ranges?
-what level of sound quality and loudness are you looking for?
An obvious choice, if you have minimal limitations is to do quite a few subs, 2, 3, 4, whatever you can fit. You get enormous cone area and allow yourself to work within the drivers linear range and stay clean. It's a simple approach that doesn't necessarily need uber $$$ subs. The only other way that works is to get massive excursion. TC Sounds does pretty well with this as most of their subs have at least a good 1" one way travel.
Equations:
SPL = 100.3 + 20log(xmax) + 40log(d) + 40log(f) - 20log®
where: xmax=piston excursion, d=piston diameter, f=frequency, r=distance to driver
units: dB, meters, Hertz
I'll note this doesn't take into account sensitivity of the woofer at X frequency and Y wattage input. It's mearly "here's the max output this woofer can do at X frequency based off max excursion, cone area, and distance from woofer. Note, xmax is linear excursion. If you know the mechanical limit(Xmech, Xsus) you can use that too if you're curious about the actual max capable excursion before damage occurs.
Sometimes it's not always excursion based limitations. At higher frequencies, it might be thermal(using rms wattage or a known "safe" wattage).
Peak SPL = ((speaker’s 1 meter sensitivity - 2) +l0*log(amplifier power) -20*log(D/4)
This could be used to determine SPL with a given amplifier or max SPL achievable for rms or other wattages. You just have to know the frequency response of the woofer and sensitivity at various frequencies. Also, this will be based off manufacturer given sensitivities which will probably not be accurate at higher output levels. These calculations are a rough guide but can give you an idea.
Also consider cabin gain. I guess the rough estimate is +12dB per octive below 80Hz, although every car will be different. It's one of those things you won't know unless you directly measure it yourself.