Tvheadend

Switch to pci-e cards. Digital devices produces a card that lets you connect 8tuners. And that is on a single pci-e lane, so lots of options to expand on newer mother boards. But keep in mind the max number of dvb devices that is supported by the linux kernel. At the moment the default is 8, but this can be changed. And tvheadend is limited to 32 at the moment, but that can also be changed.

Although there was talk of adding a htsp client to tvheadend it was not yet build. (so if you like a nice programming task .....)

An other option, that i did not try, that might work is using videolan's dvbblast to convert a dvb transport stream to ip streams per program, and then add those to your tvheadend server.

With high number of paralel streams you might hit the storage bandwidth while writing to disc. Also you might hit a cpu bottleneck if you need to decode your channels. But.... There is always room to expand ... Who knows. Run your system on a cluster :-)

that htsp client idea sounds nice, however, i don't even know how this works.
dvblast is a good idea, it works, i tested it, however i remember epg was not working, and the fact that it is not managed from the tvheadend interface is a big minus. It's pretty awkward to build a config for each channel twice in every system, compared to the simplicity of adding a mux.