No; it'll mean the same number of multiplexes, squashed into less channels. This might mean the robustness (and therefore capacity) of the muxes is increased, but that will be likely to be matched with a switch to DVB-T2, so overall there will be more capacity on those muxes, not less.
It's academic as the decision hasn't been made.
However, a multiplex occupies a whole UHF channel and all channels in the 700 MHz band are occupied. 700 MHz clearance involves reallocating UHF channels 49-60 to mobile 4G services (a loss of 12 UHF channels). Additional space could be granted on UHF channels 31-38 (8 channels) to accommodate the services lost on the channels reallocated to 4G.
Obviously, taking account of the regionally organised channel spacing the full number of multiplexes previously carried on 12 UHF channels won't fit into 8 channels. Also bear in mind that one of those channels isn't assigned to broadcasting and a further channel has been assigned to local TV. That brings the total down to just 6 UHF channels.
12 regional multiplexes displaced from the 12 UHF channels won't fit into the 6 new UHF channels. That means fewer multiplexes will be available, as stated in my original post. Clearly, they could convert the new multiplexes to DVB-T2 and merge, say, 3 multiplexes into 2. But, as I originally stated, when they start squeezing so many streams onto a DVB-T2 mux, the picture quality will suffer. We could end up with the more efficient compression of DVB-T2 being used to cram in more channels rather than to give improved picture quality.
That's just one scenario, the entire transmitter network could be organised as SFNs but as the transmitters aren't actually designed for this the distance between transmitters won't be ideal. That would probably mean that the SFNs would cover less of the population than the current arrangement. Another option considered, of course, was to only transmit the 3 PSB muxes post 700 MHz band clearance.
It will be interesting to see how things develop.