Rules for numbers and numerals:
In technical documents use the numeral: 15, 99, 20%, 5 trials, 123 million.
In nontechnical documents spell out whole numbers less that 101:
fifteen, ninety-nine, one hundred, 101, 250, 5,000
—except for percentages, years and dates, page numbers and chapter numbers:
20 percent, 2021, 14 January, page 3, chapter 10.
