The math straight up says you're wrong there. At level 5 jacks the fear duration is 1.5 seconds, and the estimated attack speed on a jack is 1.35 meaning 1.35 attacks per second, means every jack gets to hit you twice before you recover from fear assuming no tenacity, assume another shot before you get out of their range, being generous (sometimes they won't get that or even their free 2 because of the aimless walking around from fear). 95*12 is 1040 damage. Now add in your base MR. That can't even kill itemless Shaco (not exactly a tanky hero) at full at level 9 unless the Jack Shaco builds magic penetration and AP. If you catch someone that built any survivability items at all you're probably not killing them even with AP and magic penetration. And consider that that was a good beat. Pretty often the jacks don't even get their free 2 attacks.
At lower levels the jack trap effectiveness goes down because the fear duration is shorter, when compounded with the lower base damage gives you a less effective jack trap even considering that your targets have less hp.
You pretty much need 6 to solo squishy carries with traps reliably-- the AP return on jacks isn't that good so it's not like a Shaco that builds AP is going to necessarily keep up with the carries in the late game unless he's having a really good game. Now if you throw the Shaco himself in, having him camp his trap, you could easily manage it with 4, but that's a waste of Shaco's time and countered by warding. Also not much more effective than say Teemo camping shrooms.
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