A manual for writers of research papers, theses, and dissertations, 7th edition - Kate L. Turabian 2007

Special cases
Numbers
Part III. Style 20 spelling

In a few common situations, the general rule discussed in 23.1.1 requires modification.

INITIAL NUMBERS. Never begin a sentence with a numeral. Either spell out the number or recast the sentence, especially when there are other numerals of a similar type in the sentence.

Two hundred fifty soldiers in the unit escaped injury; 175 sustained minor injuries.

or, better,

Of the soldiers in the unit, 250 escaped injury and 175 sustained minor injuries.

When spelling out numbers over one hundred, omit the word and within the term (not two hundred and fifty).

SERIES OF NUMBERS. Ignore the general rule when you have a series of numbers in the same sentence that are above and below the threshold, especially when those numbers are being compared. In these examples, all are expressed in numerals.

Of the group surveyed, 78 students had studied French and 142 had studied Spanish for three years or more.

We analyzed 62 cases; of these, 59 had occurred in adults and 3 in children.

If you are discussing two sets of items in close proximity, ignore the general rule and, for clarity, spell out all numbers in one set and use numerals for all numbers in the other.

Within the program, 9 children showed some improvement after six months and 37 showed significant improvement after eighteen months.

ROUND NUMBERS. Spell out a round number (a whole number followed by hundred, thousand, hundred thousand, million, and so on) in isolation (see 23.1.1), but give several round numbers close together in numerals. You may also express large round numbers in a combination of numerals and words. (See also 23.1.4.)

They sold 1,500 copies in the first year and 8,000 in the second.

These changes will affect about 7.8 million people in New York alone.