It is a common statement that you use only 10% of your brain. This is obviously wrong. When did you ever hear on the evening news that a shooting victim was hit in the head but luckily the bullet landed in the 90% of his brain that was not used?
There is one case where the ENIAC computer could be said to follow the 10% rule. This was the first programmed electronic digital computer. Its program was stored in panels of external rotary switches with instructions like Talk Register 1 and Listen Register 2 (TR1, LR2) to transfer a number. It had a light display with the digits painted on the bulbs in an array:
9 9 9 9 9 9 9 9 9 9 8 8 8 8 8 8 8 8 8 8 7 7 7 7 7 7 7 7 7 7 6 6 6 6 6 6 6 6 6 6 5 5 5 5 5 5 5 5 5 5 4 4 4 4 4 4 4 4 4 4 3 3 3 3 3 3 3 3 3 3 2 2 2 2 2 2 2 2 2 2 1 1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0 0 0
To represent a ten-digit number, only one light in each column would be on at a time. That would be 10% of the lights were lit. You could say that it was using 10% of its display but even then it had a lot more circuits that were in use.
It can be seen in brain scans that most of the brain has some activity all the time and it can vary in intensity. There is no 10% rule. The number of parts or cells that are in use is not as interesting as the number of states or connections that are possible.
The ten-digit example display above has 10,000,000,000 (ten billion) possible values. Instead of saying 10% of the lights are on, we can consider that only one out of ten billion states is being used. That is a very inefficient decimal display. If all the lights were used as a 100-bit binary display, in which every light has a 50% chance of being on, that would have 2 to the 100 power states, or 1,267,650,600,228,229,401,496,703,205,376. Rather than 10% of the parts, think of the 2^100 states or possibilities.
We use a very small percentage of the possible states or connections in our brains, which implies we can learn much more than we currently know. You even had room for this gem!