Definetly diminishing returns. Let's say you have one 18" driver, and you add a second identical one with same watts- you gain +6db. Nice.
Now let's say you want another +6db, so this time you need to add 2 more. You need to double it. Now you have 4 subs, and another +6db. Nice. Still worth it.
Let's say you are addicted and you want another +6db, now you must add (4) more. But you get the SPL. Nice. Possibly still worth it.
Next step is add (8) more for 16 total - Then 16 more for 32 total. It starts getting silly.
If you have 6 or 8 subs already, adding one or two more doesn't do that much. It's like only1 or 2db.
If you have weak subs or small subs then adding a more powerful or bigger one can make more difference, I was just assuming you add the same things. In Tim case for example, he had some UXL 18" drivers, which are nice subs. But adding those two 24" will still make a difference because output wise those 24" are like 2.3 UXL drivers each. So it's like adding 5 more .... Which is rather noticeable.
Mark is using the same 24" woofer in his new design I think. Because Nick is selling a lot of OEM on those is the primary reason the price increased to 1599$ each on the drivers, even though you can routinely buy them cheaper. I think SI just wants to let's the oem customers charge high prices using them without making it look bad, or make it justifiable.
You'll be able to add more subs, but you might want to try to cut cost with some DIY. Otherwise it's gonna get silly expensive. It might not be worth it depending on costs.