Style guides do not entirely agree on which numerals should be written out and which shouldn’t. All major style guides do agree that a number beginning a sentence should be written out or the sentence be rewritten so the number does not come first: prefer Eighty is old enough to 80 is old enough.
For all other numbers, The Chicago Manual of Style encourages spelling out whole numbers one through one hundred and round numbers (e.g., one thousand) in non-technical writing, Garner and APA style suggest spelling out one through ten, while the MLA Handbook prefers writing any number that can be written in one or two words (thirty-three and ten thousand but not 10,033). In this rare case I prefer the MLA approach. Written numbers make for cleaner writing. Numerals stand out in blocks of text possibly more than even italicized phrases. The eye sees the different shapes and treats them as separate from the reading. If written, the number becomes part of the text and does not call undue attention to itself. I thus suggest writing out one- and two-word numbers when appropriate. For those who wish to remain with the general one through ten rule, I encourage not to write two separate numerals beside each other: prefer I have 13 fifty-page books to I have 13 50-page books. For dates, 10 Nov 2019 reads more readily than Nov 10, 2019, where two neighboring numerals may slow down comprehension.
Scientific values, dates, monetary measurements, and percentages are often not spelled out. Percentages can be written out, though Garner advises against this, if doing so better suits the work’s style. American English prefers percent as one word (and the phrase at . . . percent) while British English—and, archaically, The New Yorker—prefers per cent as two words (and the phrase on . . . per cent).