Each sub will add around 6 db SPL. All things equal, which is rarely the case (same excursion, box size, placement, amp power etc).
A 12" sub is around 40% more surface area than a 10" and a 15" is just over 100% more or double. So more or less two 10's is sort of like one 15. Four 8" is like one 15". Six 6 inch woofers is like one 15".
Box volume is a big factor too so a box which is half the size of another with the same size woofer will have 3 db less SPL for the same bandwidth (bear in mind, a bigger box will of course use a more efficient driver!).
Ports are also a factor - a port can add 3 dB SPL but at the expense of making the subwoofer have a high group delay and poor transient response.
Amplifier is also a factor - doubling the power can add 3 db SPL (provided the driver can take that power - very long throw!!). So a 200 watt sub with 15" woofer is kind of like an 800 watt sub with a 10" woofer or a 3200 watt sub with an 8" woofer.
The worst part about all this is that for each octave you go lower in frequency you need EIGHT times the efficiency to maintain the same SPL output - so you are on the right track - ignore those 1 foot cube aesthetic looking subs as a foot rest is really all they are actually good for!
So you can see that SPL does not come easy...in some cases big really is better!
The only trick you have is that corner placement will give you an extra 6 db compared to wall placement...often this is at the expense of problematic modal problems.
Finally these are all generalizations...
A 12" sub is around 40% more surface area than a 10" and a 15" is just over 100% more or double. So more or less two 10's is sort of like one 15. Four 8" is like one 15". Six 6 inch woofers is like one 15".
Box volume is a big factor too so a box which is half the size of another with the same size woofer will have 3 db less SPL for the same bandwidth (bear in mind, a bigger box will of course use a more efficient driver!).
Ports are also a factor - a port can add 3 dB SPL but at the expense of making the subwoofer have a high group delay and poor transient response.
Amplifier is also a factor - doubling the power can add 3 db SPL (provided the driver can take that power - very long throw!!). So a 200 watt sub with 15" woofer is kind of like an 800 watt sub with a 10" woofer or a 3200 watt sub with an 8" woofer.
The worst part about all this is that for each octave you go lower in frequency you need EIGHT times the efficiency to maintain the same SPL output - so you are on the right track - ignore those 1 foot cube aesthetic looking subs as a foot rest is really all they are actually good for!
So you can see that SPL does not come easy...in some cases big really is better!
The only trick you have is that corner placement will give you an extra 6 db compared to wall placement...often this is at the expense of problematic modal problems.
Finally these are all generalizations...