I don't know for sure, but I've heard 8, but the variables such as: what size table, what size pockets, what kind of table could make a LOT of difference ???
...I was absolutely shocked that he failed to get out that rack.
I had an opponent on a valley cougar and we were using the oversized ball and I made 7 on what could be described as only a medium hard break (ironically for control) and I was frozen to the 9 with no shot on the 7 (lowest remaining) ball.
I doubt that this is any type of record; I've heard of 8 from credible sources about Danny Medina in his 20's or early 30's on more than one occasion and I have witnessed a "D" player break in the same 5 balls 3 racks in a row.
The 9-ball break is very random, but less so with magic rack, sardo, slug Dr. and their kind ...
I've done 6, once, and didn't get out. Had the 6, 7, and 9 left on the table. Was hooked by the 9 ball.
I've seen 6 a couple of times. All on bar tables. Guy I was playing the other night made 5 twice on the break. Most I made the other night was 3 or 4, really don't pay much attention. Just remember the 6 because I was hooked which sucked.
I've made 6 several times with a couple of the other balls lagged in the pockets, so I could see 8 happening. Live rails and a hard break will do it. I have made 6 in 8 ball and they were all stripes that left me two stop shots for the fastest out ever. My opponent and I just looked at each other like "WTF". There is enough pool played anything could happen. I knew a fella who broke and knocked himself out, wish I would have seen that one. Some how he slipped and hit his head on the table after a break shot.
I have seen 6 made a lot of times over the years but strangely I have never seen 7 made and that is having seen tens of thousands of racks broken. Although, probability would dictate that at some point someone must have made all 9.
I saw 7 made once out of 45 years of pool, the guy missed a table length difficult cut on the 8 ball and did not win the game. His name was Joe DiVito here in Columbus Ohio. He was probably a high C player.
The most balls I made on the break in 9 ball with a witness was 5. The most I made without a witness was 10. The cue ball went 11 rails and
made the 9 and then followed it in.
On a Valley bar box during league I made 5 balls on a 9 ball break...and the cue ball was snookered so I couldn't get out, lol. What luck!!!
Most 9 on the snaps I've made in a row was 3...Valley bar box. Still amazed that ever happened. Guy must have racked the exact same way each time. 9 rolled into the same back corner pocket untouched each time.
I think the pool gods have an unwritten rule where if you make too many balls on the break you will be left with no shot. I have made 5 maybe twice and 4 numerous times, but rarely have any kind of a shot. Its a conspiracy I tell ya!
On the big table I won a game of rotation on the break once. Rotations needs 61 points to win. Also made 7 balls on the break playing nine ball on the bar table but unlike some others here I did win that game.