Here on Quora the answer provided includes a tidbit about the 88s in africa. It states that it took, on an average 20 rounds from a 88 to kill a Matilda.
Now the Matilda is a very cramped tank, and the 88 is fairly accurate. Was it really that hard to knock out a tank?
The Quora answer is a pretty solid response, but it shouldn't be interpreted as 20 hits from an AT gun to knock out a tank.
Although short by modern standards, tank and anti-tank guns typically fired at reasonably long ranges, perhaps 200-700 meters or longer. An 88mm actually shoots out to 2000-3000m or so, but practical considerations like terrain and conditions mean that most targets won't be able to be seen until much closer. Even at only a few hundred yards though shooting a moving target is quite difficult, as well as accounting for the trajectory of the shell and correctly determining range (although less of an issue for the flat trajectory 88mm.)
Also, AT guns don't always fire with the intent of making every shell count, sometimes they are just suppressing infantry or trying to keep armored vehicles pinned down. Sometimes they are shooting at a log or bunker that happens to look like a hull down tank in the rangefinder. Sometimes they shoot at a knocked out tank 8 times and don't realize it already burned out an hour ago. All the fog of war stuff that isn't like what happens in a video game.
This shouldn't seem strange at all when you compare it to rifle fire vs. infantry casualties. Tens of thousands of rifle rounds are fired for every enemy casualty, although it is usually a fairly serious injury to for a human being to be hit by a single rifle round.
I couldn't see the quote reading through several of the posts. One thing I'll ask is the definition of "kill". A single AT penetration could not hit any crew but panic them enough to bail out. Or it damages the tank so that it cannot move and is stuck there in the open and the crew bailed. Or the gun is damaged and it cannot fire back and the crew bailed. Or it killed some/all of the crew so the tank. Now in all cases this tank is no longer combat operational - is that a kill?
However this tank could be recovered and quickly repaired and brought back into service. So they often fired at a tank repeatedly until it burned to ensure it could never be repaired and re-used by the enemy. If you've got a later Sherman with wet-stowage ammo that inhibits ammo cook-off then it might take multiple shots before it goes up in flames. So yes it might take 20 shots to 'kill' a tank if the definition is to completely destroy it beyond salvage.