Only the one! It would be the one who had his bicycle stolen as a youth, and noticed a boxing poster at the police station when filing a complaint. Angry about his bicycle being stolen, the young boy named Classic Clay followed the address on the poster to the boxing gym. Clays purpose to learn boxing was just in case he ever meets the man who stole his bicycle. Without this random event of bad luck, there is no Ali.
One has to wonder how the family dynamic would effect the attitudes and personalities of the septuplets. This could be a key factor... :think
The other septuplet would have the same experiences that made Ali who he was. They wouldn't get as much attention from their mums, so they'd be very different people, not as mentally quick. Septuplet's are also likely to be smaller so no HW champ even if they excel at boxing. Would they get as much food to help them grow into strong young men? But yea they obviously all have potential to be ATGs