a. The novelist the number of whose published novels we did not know started speaking.
b. The novelist the number of novels published by whom we did not know started speaking.
c. The novelist the number of whose published novels was not known to us started speaking.
d. The novelist the number of novels published by whom was unknown to us started speaking.
There was a group of novelists. We did not know how many novels this one particular novelist had published. That was the novelist who started speaking. We are basically defining the novelist who started speaking. He was the one who had written a number of novels but we didn't know how many.
Are the sentences a-d grammatical? Do they break any rule of grammar? Would they ever be used? Are they comprehensible?
I don't think they are ungrammatical, but I suspect they are just too convoluted to be used.