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When you write in English, you should spell out numbers under 10 in formal writing. For example, do not write something like, “I ate 6 donuts.” It is technically correct to write, “I ate six donuts.”

For 10 and above, it is fine to use numerals, e.g., 10, 11, 12, 13…

So it is correct to write, “I ate 15 donuts over the weekend.”

Obviously, headlines, chatting, and messages etc. can be exceptions to this rule because of their limitations in terms of space.  Also, when a number immediately follows the word it modifies (for example, “level,” “grade” etc.), it is acceptable to use numerals for numbers below 10.

Also, we use commas every three zeros in large numerals. For example “I ate 1,000 donuts.” not “1000.” This is because without commas, it is very hard to read large numerals.

Once you get up to one million, we usually switch back to spelling out numbers again. For example, we usually write, “10 million,” not “10,000,000.”

Additionally, you should never begin a sentence with a numeral.  It is incorrect to write a sentence like, “1996 was a great year for me.”  You should instead change the numeral to a word, or rework the sentence, e.g., “One of my best years was 1996.”