RULE: Numbers 1-10 are spelled out; 11 on up are not

See? That isn't a “rule”, it’s an opinion, a choice. Some claim to spell out numbers “ten and above”. Most fiction writers are told to spell out every number. I ask: even eleventy trillion? My Style Manual choices: use numerals for measurements (miles, feet, inches, dates). In fiction, write out round numbers up to 100 (forty),  switch to numerals for specific amounts (75). I do like to spell out three million, but I’ll use figures for $3 million. See? It depends on your preference — and what you're writing.