Complex sentence: Of the 158 delegates with speaking rights, 39 participated in the debates--much-lower participation than at the 5th Congress, in which 98 of 137 delegates with speaking rights participated.
Split into simpler sentences: Of the 158 delegates with speaking rights, only 39 people participated in the debates. This is far less than at the 5th Congress, in which 98 out of the 137 delegates with speaking rights participated.