How many 6.5in driver equals one 8in driver

Here is an example of driving four 5in. driver.  Basically it is two in series and then each two in parallel with the other.

It is from Zaph, a very well regarded speaker designer.  You can see below how the drivers are being driven.  The speakers are MMTMM layout.  In this way, you can have an overall impedance of 8ohm.  If you drive them all in paralle, the impedance would be 2ohm which is too low.

The kit is available at Madisound.

https://www.madisoundspeakerstore.com/2.5-way-speaker-kits/zaph%7Caudio-za5.5-tall-tower-2.5-way-pair/

Enclosure drawsing:

https://www.zaphaudio.com/ZA5/ZA5.5tt.pdf

I have yet to hear a speaker w/ multiple small woofers that can match a big box w/ big woofers assuming both are  of good quality. The whole concept of insisting on narrow speakers to minimize diffractions & improved imaging is questionable which is mostly why speakers use multiple small woofers. 

Probably cheaper to build the cabinet too.

"to hear a speaker w/ multiple small woofers that can match a big box w/ big woofers” - in 40yrs I haven’t seen any small driver systems, to match.. only small drivers sound good are in my headphones .. the best LF drivers I’ve heard were 18” in large cabinets..