I looked around a bit and it seems people say 1-4%, but that there isn’t much research. I also think it’s inherently hard to measure, because it’s possible to have it without realising it. The only person I know who has it didn’t realise it until he was almost 30. I assume that people with aphantasia will find visual mnemonics harder, but I think you are mistaken when you make it sound that it’s either visual mnemonics or rote memorisation. I would say most mnemonics work very well without visualising anything. I don’t need to visualise a horse riding my mum to remember the character 妈/嗎, as the idea will appear bizarre and therefore memorable anyway. People with aphantasia have no problem imagining that two things typically don’t go together and would therefore work well as a mnemonic. You can also use other senses, even though I agree that visual mnemonics are the most powerful, assuming that you don’t have aphantasia then. But even if you do, rote learning is certainly not the best way.