Although style guides do not 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 Nineteen-eighty is an even year to 1980 is an even year.
For general 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; Bryan A. Garner and the Publication Manual of the American Psychological Association suggest spelling out one through ten and numerating everything else; and the MLA Handbook, similar to The Chicago Manual of Style, prefers writing any number that can be written in one or two words (thirty-three and ten thousand but not 10,033).
I suggest following the MLA Handbook unless writing under another style guide. Its approach leads to fewer numerals in the text and a better lexical division between characters and numbers. Written numbers make for smoother reading—numerals stand out in blocks of text possibly more than even italicized words. The eye sees the different shapes and treats them as separate from all else. When written, the number becomes part of the text and does not call undue attention to itself. For those who wish to remain with the one-through-ten rule, avoid placing 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 cleanly than Nov 10, 2019, where two neighboring numerals may slow comprehension.
Scientific values, dates, monetary measurements, and percentages are often not spelled out. See units for appending units to numerals.