No, John, the maximum number is 20.
Try and build a 2.1 config file and you will find it out. Just add manually more filters and you will get a 3x20 filters config.
With 4 speakers maximum per channel is 15 filters.
More generally: maximum number of filters per channel is 20; maximum number of automatically calculated filters per channel depends on the number of active channels: here are the maximum ratios betweend active channels and max auto filters: 2/20 - 3/20 - 4/15 - 5/12 - 6/10 - 7/8 - 8/7
If you don't believe me, just try this out and see what happens.
With 8 active channels, Mconfig will never add more than 7 filters.

My automatically-calculated target decay time is more than 800 ms, due to the large and odd dimensions of my listening room, and highly lively walls and ceiling. And - even at 800 - no filter is created at all.
To have Mconfig creating more filters I have to get as down as 400ms. And even then it creates filters only as described above.

I simply would like to have my system use room correction only with front speakers and sub.
I can do this manually but it is very time-consuming.
Any suggestion on how to manage my unit and / or Mconfig to reach the above mentioned result?