Rules for writers, Tenth edition - Diana Hacker, Nancy Sommers 2021

Follow the conventions in your discipline for spelling out or using numerals to express numbers
Numbers
Mechanics

MLA style uses numerals only for specific numbers larger than one hundred: 353, 1,020. Spell out numbers one hundred and below and large round numbers: eleven, thirty-five, fifteen million.

APA style uses numerals for all but the numbers one through nine. Spell out numbers from one to nine even when they are used with related numerals in a passage: The survey found that nine of the 157 respondents had not taken a course on alcohol use.

If a sentence begins with a number, spell out the number or rewrite the sentence.