Within a margin of error, zero people can remember 20 16-character random alphanumeric passwords. Therefore it is only possible using some sort of password manager, whether it be something like 1password or an old-fashioned notebook.
You need to specify your margin of error. ± the full population of humans on Earth is "a margin of error".
I may be an outlier, but I certainly remember 10+ 20-25 character random full-printable-ASCII passwords, some of which don't let a password manager handle them, others which I don't want to have in a manager. And then there's my password manager master password, which is close to 70 characters long.
And I have shitty memory—I wouldn't be able to remember what happened more than a few days ago if my life depended on it.
Nitpick nitpick nitpick: "margin of error" without any value effectively means "the following value has no meaning at all", as the margin of error is unspecified.