That's... not relevant in any way. The number 20 is still one number, with two digits. If the discussion were over version 17456295702 or something else with a lot of digits to remember, *then* it would be relevant.
(PS. The idea in question is, of course, utter bollocks. For supporting evidence, see the 3 billion people who happily memorise lots of 11-digit telephone numbers, for example)