Complex sentence: Though no details of the meeting survive, Charles IX and his mother took the decision to eliminate the Protestant leaders, Holt speculates perhaps "between two and three dozen noblemen" who were still in Paris.
Split into simpler sentences: Though no details of the meeting survive, Charles IX and his mother apparently made the decision to eliminate the Protestant leaders. Holt speculated this entailed "between two and three dozen noblemen" who were still in Paris.