Numbers as words
As a general rule, in running text, spell out the numbers one to nine, but use figures for numbers 10 and up. Examples:
The YouTube channels of seven European and Middle Eastern universities will be accessible on the EDU platform.
The course load will range from 60 to 90 credits per academic year.
This notwithstanding, when a passage contains both, try to be consistent. For instance, if a number in a given category must be written in figures, use figures for all the numbers in that category. Examples:
Nine of the twelve research groups recognized by the AGAUR are emergent groups [not Nine of the 12…].
He is the author of 3 books, 46 refereed journal articles, 122 refereed conference papers, and 16 book chapters [not …three books, 46…].
Such exceptions should be applied in the passage in question but need not extend to the text as a whole.
Exceptions are also made for times and dates, which, even in running text, are nearly always referred to with figures. Examples:
The conference will take place in Barcelona on 1 and 2 July.
The group will meet on Thursday from 5 p.m. to 8 p.m.
Remember, both cardinal and ordinal compound numbers from 21 to 99 take hyphens when written as words. Examples:
twenty-one
thirty-third
In non-technical texts, use words when there is no way to avoid beginning a sentence with a number. Example:
Fifteen out of sixteen statistics are made up on the spot.
That said, another solution can often be found. Example:
Some 15 out of 16 statistics are made up on the spot.
The year 2011 was a dramatic one.