This Grammar Monster article advises that one should not start a sentenced with a number in figures, except possibly when it represents a year, but gives no reason except that doing so is "untidy", which does not seem much of a reason to me.
"Three Tips for Starting a Sentence with a Number" from Erin Wright give much the same advice, but says that:
starting a sentence with a number can be confusing or disruptive for readers—doing so is generally grammatically acceptable if you follow the three guidelines outlined below
Those guidelines being to spell the number out, reword the sentence so the number is not at the start of the sentence, and to consult the relevant style guide. The article mentions that:
Our four primary style guides (i.e., The Chicago Manual of Style, The Associated Press Stylebook, the Publication Manual of the American Psychological Association, and the MLA Handbook from the Modern Language Association) all agree that numbers should normally be spelled out at the start of sentences.
This tip from the MLA Style Center says:
Since you should never begin a sentence with a numeral, you should first try to reword the sentence. If you find it unwieldy to reorder your words, spell out the number.
But it also give no particular reason for the rule.
"Can you start a sentence with a number?" from TopContent advises:
When writing, it is always important to be able to have a flowing and readable structure. One of the fastest and most noticeable ways to mess this up is by beginning a sentence incorrectly. Both new and experienced writers often struggle with what they can and cannot start a sentence with, often having been taught one thing and sticking to it.
The post goes on to recommend some of the same solutions, with some added specifics of not using a number at the start of a sentence, even spelled out, if it is followed by a unit of measure, includes a decimal point, is a percentage, or is a large number. The post also suggests consulting a style guide.
This suggestion of "flow" seems a bit more to the point to me.
Finally "Seven (or 7?) Tips On Starting A Sentence With A Number" from Michael Blumfield, posting on LinkedIn, says:
We’ve established structures for signaling to readers that we’ve reached the end of one sentence and we’re beginning another. This is so built in to our way of writing that we don’t think about it:
- Use a period to say you’re done with one sentence.
- Put a space – or in the old days, two spaces – between the period and the next sentence.
- Capitalize the first letter of the word beginning the new sentence.
Looked at that way, spelling out the number is the equivalent of that capital first letter. It’s a way of saying, “This is the beginning of a sentence.”
Without the contrast between the word and numeric form, there's no way to indicate that shift. You could say that the period and space are enough, but if so, why not start sentences with a lowercase letter in the first word?
The post goes on to say that the rule is outdated, and in many cases can now be ignored.
I find this rationale -- that a digit cannot be capitalized and so makes the start of a sentence less clear, persuasive. I had long known and followed the rule, but with no clear idea why. This gives what seems to me a good reason for the rule, better than that for many rules of English usage.
We also see that there is wide and varied agreement on the basics of the rule, which is enough to make it clear that this is a customary part of English usage.
