How many 6.5in driver equals one 8in driver

"How many 6.5in driver equals one 8in driver"

multi-diver (at one band, LF/MF/HF) speakers have a problem to sum different sound waves across frequencies without frequency, phase and linearity distortions. thus, it’s not possible to replace one good large driver with multiple drivers, and to have same SQ performance. 

I don't know what you are attempting to convince yourself of, but if you are looking for an equality of sound, then you cannot determine that by the size of the driver alone. There are far too many variables involved, including materials used, what type of driver (eg, long throw), and especially the cabinet size, dimensions, shape & ratios, ported or sealed cabinet, stand mount or floor mount and the position of the driver with respect to room surfaces.

as an example, yesterday I heard a pair of Fyne F1-5 speakers with a 125cm driver (5") that went as solid in the bass region as a KEF R3 Meta with one 5" aluminum midrange driver and one 6-1/2" hybrid aluminum woofer. The Fyne was twice the price of the KEF but held it's own (and then some) with the larger drivers in the KEF.

Spandl says: “The trio of 125mm bass drivers give the authority of a single 250mm woofer, but in the same slimline cabinet.”

From Spandl, a designer at Acoustic Energy.

https://www.hifichoice.com/content/acoustic-energy-ae520

+1 @westcoastaudiophile   I have found the quality of the bass from a single driver better than when multiple woofers are used (whether subs or in the same cabinet).  

All of these calculations omit the fact that an 8" driver will typically have more excursion than a 6.5" driver, and it is the volume of air moved, not the cone area, that determines how loud they can play.