I have 22 tuners and a scheduled installation on June 12.
DECA can support up to 16 tuners so you'd need one SWiM-16 and one SWiM-8, you could in theory have a single dish feed the SWiM-16 and then cascade to the SWiM-8 for the remaining equipment.
DECA can support up to 16 devices, so if all 22 tuners are DVRs (11 devices + 1 broadband), you could, in theory, use DECA. The issue of course comes with connecting the SWiM-16 & SWiM-8 such that DECA can traverse both SWiMs.
veryoldschool has posted a method by which this might work (in theory), but I don't think anyone has actually attempted it.
Without this special (untested/unapproved) connection, you would have two separate DECA clouds and DVRs on each cloud could see each other plus if both clouds are connected to your home network, you'd see receivers on the other cloud via your home networking.
If your least-used DVRs are connected to the SWiM-8, you still may be fine with the upgrade.