When Antony and Octavius have a parley with Brutus and Cassius on the battlefield just before the battle at Philippi in Act 5, Scene 1 of Julius Caesar, both Antony and Octavius offer some information about the death of Caesar. Antony says:
Villains, you did not so when your vile daggers
Hacked one another in the sides of Caesar.
You showed your teeth like apes, and fawned like hounds,
And bowed like bondmen, kissing Caesar's feet,
Whilst damned Casca, like a cur, behind,
Struck Caesar on the neck. O you flatterers!
Octavius swears never to put up his sword
Never till Caesar's three and thirty wounds
Be well avenged, or till another Caesar
Have added slaughter to the swords of traitors.
Evidently Shakespeare got the thirty-three wounds from a translation of Plutarch. Shakespeare's description of the assassination of Caesar in Act 3, Scene 1 came mostly from his own imagination. He writes that the various assassins crowded around their victim and all took turns stabbing him
They stab Caesar. Casca first. Brutus last.
It may have been historically true that Brutus was the last to stab his friend Caesar, but certainly not that Caesar said, "Et tu, Brute" as he died. The death of Caesar in the play seems anticlimactic after all the plotting building up to it. This was no doubt intentional. Action on a stage always looks unrealistic. Shakespeare wanted the turning point to be Antony's funeral oration, not Caesar's assassination.