I have witnessed seven aces. But to be an absolute stickler for the truth I only saw two go in the hole as the bottom of the flagstick was shielded on four occasions. The seventh I didn't see go in because it was struck directly at the sun in late evening. My friend who struck the shot and I searched short, long, left and right, before I walked over and looked in the hole. The ball had gone in on the fly and had scalloped out a portion of the cup about the size and shape of the top of a thumbnail. When I announced the ball was in the hole, my friend, unable to believe he had actually done it, threatened me with grievous bodily harm because "That's just not something you joke about." Once he saw the ball in the hole he mellowed out and as it was the ninth hole we adjourned to the clubhouse where he eagerly bought a round for everyone present.
Another ace I heard hit the flagstick on the second bounce.
And once, while waiting my turn to putt on a green adjacent to the tee of a short par three, I saw a total stranger make an ace and then whoop, holler, and gyrate in a manner that threatened his life.
Finally, my first hole in one came on a blustery November day at a discount version of a volcano hole that was playing into a copse of trees by the Ohio River. I announced that I was going to try and play a knockdown shot with my 9 iron to keep the ball under the considerable wind. Instead of a knockdown shot, I bladed it and the ball, never more than ten feet in the air struck the mound at the front of the green and then scurried through the leaves and dead centered the pin with a clank. One of my playing partners said, "I think it's in the hole." Another said, "Or in the river where it belongs."