me too. I am on Cubase 9.5.10 on Win 7, but even restarting does not give me more than 8 side chains in the sends, and that is across the entire project.
So, even duplicating the track of the side-chain kick (or a group channel fed by the kick) which in theory should give me 8 more, but when I check the sends and look through the side-chain section, only 8 show up.
For making EDM music this is beyond useless. But for EDM makers there is a solution - LFO Tool plugin from Xfer (50 USD), you just manually tweak it to pump the track like the sidechain would. (Also TAL Filter 2 is free ( as is Reaper… just sayin))
EDIT: ten minutes ago I was cussing Steinberg but tbh if this hadnt happened I never would have found TAL Filter 2 which gives waaaaay more control over the pumping side-chain effect. Just takes a few minutes to work out the way to place the filter over the sound to get the desired effect and then much more granular tweaking than the side chain compressor offers, just stick the original compressor right after the TAL and bobs yer unkle. noice.