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I know you have to write out numbers less than 10. However, if you're quoting a source that doesn't do so, do you need to write [sic] after each number less than 10 that isn't written out?

For example, should “nationwide, 25.4% of students got 8 or more hours of sleep on an average school night” be written as “nationwide, 25.4% of students got 8 [sic] or more hours of sleep on an average school night”?

I'm using the MLA format.

No, because whether to write numerals or to spell them out is a point of style, not grammar.

Are you using a particular style guide that indicates you should do this? Otherwise, no, don't use sic. Using it here would lead the reader to believe that 8 is the wrong number and maybe the author actually meant 10 hours.

In APA format for example, sic is not used with things like British spellings, even if they can't be used outside of quotes.

Actually, it is correct to use the number 8 here instead of the word according to APA, since it refers to an exact quantity of time. See my answer here for more info.

I'd advise using 'sic' only when the reader might otherwise doubt whether a word or phrase was being quoted correctly.

As a rule of thumb, [sic] should hardly ever be used at all, and certainly not in this case, as other answers have explained.

The use of [sic] implies that the person being quoted made a mistake. It further implies that this mistake was obvious enough to require acknowledgement. But most of the time, acknowledging a mistake is a distraction from the subject matter, unless your purpose is to impugn the writing. In a wide array of cases, there are alternatives to [sic], which will generally hold the reader's attention better than interrupting them with a big "Hey, spot the error!" sign:

• For simple typos, you can replace the incorrect letters or words with the correct form and enclose it in square brackets.


• When transcribing speech, omit filled pauses and other speech disfluencies altogether, and transcribe what the speaker intended to say. You should only transcribe the spoken word verbatim if a disfluency is somehow notable in its own right, or if the speech is so meandering that you cannot tell what the speaker intended.


• Older documents such as the US Constitution sometimes use capitalization or punctuation in ways that differ from modern grammar. In this case, it's usually enough to write "[capitalization in original]" at the end of the quote. If you'll be quoting the same document repeatedly, you can write "All quotes from the US Constitution are capitalized as in the original."


• If the writer is following different rules of style from your own (as in this case), or is writing for a different dialect of English, tough luck. Leave it verbatim and do not mark it, unless you believe it will be difficult for your audience to understand. In that case, you should change the material as little as possible to make it comprehensible (and always enclose changes in square brackets). Alternatively, provide an explanation or paraphrasing outside the quotation.
  
  
  • Some English dialects have a rather parochial attitude towards other dialects. For example, native speakers of British and American English sometimes label Indian English as incorrect. If you fear your audience may have this reaction towards the material you wish to quote, it is particularly important to avoid the use of [sic], which may be perceived as insensitive. Instead, you should explain the material's origin and indicate, outside the quote, that it is verbatim.
  
  


• If the error is severe enough that you are unable to determine what the writer meant, then you will probably discuss that fact outside of the quote. There is no need to use [sic] if you're already talking about the problem anyway. A reader who doubts your transcription in this case will continue to do so even if you do write [sic], so it provides no benefit.