Acts 23:13

‘And they were more than forty that made this conspiracy.'

The size of the conspiracy comes out in that ‘forty' men were involved. Such a number would be needed in order to keep the attention of the Roman guards who might be expected to escort the prisoner, while the assassination was taking place. And the assassination had to take place in the short time before Paul reached the Sanhedrin. Forty is regularly a number connected with judgment and trial (forty days of rain at the Flood, forty days of Goliath calling on Israel to fight him in the confidence that they would not, forty days of Elijah in the wilderness), and with the giving of the Law (forty days in the mount twice over, without food and drink). Perhaps they (or Luke) saw it as symbolic of their aim, to avenge the breaking of the Law.