Under D&D rules, a dagger does 1d4 base damage. The average human has a Strength score of 10, adding no bonuses. Several of them, due to the military background of many, likely had strength or dexterity scores of 11-14. But only two or three, and quite a few would be frail with old age, sinking to 8-9 strength. All in all, we can only add a total of +1 damage per round from Brutus.
An estimate of sixty men were involved in Caesar’s actual murder. Not the wider conspiracy, but the stabbing.
Julius Caesar was a general, which is generally depicted as a 10th level fighter. Considering his above baseline constitution and dex, weakened by his probable history of malaria, epilepsy, and/or strokes (-1 dex modifier), and lack of armor at the time of the event, he would likely have something along the lines of AC 9 and 60 HP. The senators would likely hit him roughly 55% the time.
So the Roman senate had a damage-per-round of 66, more than enough to kill Caesar in one round even without factoring in surprise round advantage.