9 things is considered a standard for working memory (kind of like processor registers), for people with ADHD it's even less - 3-5.
Try writing a number from one piece of paper to another. If it's more than 7-9 numbers, you won't do it in one shot, unless you spend extra time memorizing it.
That can be increased quite a bit with practice. But it's also not important. It's just the cache memory -- it isn't the limit of what can be learned and recalled.
It is a limit on what you can reason about without a piece of paper.
I’m proficient at math, but my working memory is around 6, so I cannot add two three digit numbers to each other in my head (unless I see numbers to be added in front of me).
Try writing a number from one piece of paper to another. If it's more than 7-9 numbers, you won't do it in one shot, unless you spend extra time memorizing it.