The 12s and 8s use the same class of amp (BASH) and both are ported
This is an oversimplification. The Amplifier has nothing to do with the acoustic performance of the Subs. Neither does the fact both are ported.
What actually matters is the frequency at which each Subwoofer rolls off and the tune of the cabinet and port. Following on to the topic above about how the 8" will limit the performance of the 12" Subs, the most simple example I can offer is that the 8" will likely have a lower SPL capability than the 12". If you fine tune your system to not over drive the 8", you will be limiting the 12" and vise versa: if you fine tune to the 12" you will almost certainly be overdriving the 8".
From everything I've read, 2 subs are better than 1, and 4 subs are better than 2.
Yes and no.
The key aspect goes back to understanding acoustics. You really want matching Subwoofers when you add multiple Subs. The performance of each Sub is identical and you do not introduce complications in individual acoustic performance to the already complicated aspect of room acoustics. Managing different phase and time alignments as well as multiple tuning frequencies of the Subs is a common headache of many who use multiple Subs that do not match.
By matching them, you just have to get them to play nice in the same room which is also fairly advanced.
Most people doing this are using additional DSP and measuring devices like MiniDSP 2x4 or Flex and Room EQ Wizard (REW) with a calibrated measurement mic to measure and adjust for all the issues of correcting such a complex system.
It is much more complicated than just dropping a Subwoofer or four in a room and turning it/them on.