Here is what the Chicago Manual of Style has to say (in part):
9.1Consistency and readability
This chapter summarizes some of the conventions Chicago observes in handling numbers, especially in making the choice between spelling them out and using numerals. Consistency must sometimes give way to readability. Even in scientific and financial contexts, where numerals are used far more widely than in the humanities, they can never totally replace spelled-out numbers, and few readers would want them to. The following guidelines apply mainly to general works and to scholarly works in the humanities and social sciences.
Among the factors governing the choice between spelling out numbers and using numerals are whether the number is large or small, whether it is an approximation or an exact quantity, what kind of entity it stands for, and what context it appears in.
9.3Chicago’s general rule
In nontechnical contexts, the following are spelled out: whole numbers from one through one hundred, round numbers, and any number beginning a sentence. For other numbers, numerals are used. For the numerous exceptions and special cases, see throughout this chapter and consult the index. For hyphens used with numbers, see 7.90, section 1.
Thirty-two children from eleven families were packed into three vans.
The property is held on a ninety-nine-year lease.
The building is three hundred years old.
The three new parking lots will provide space for 540 more cars.
The population of our village now stands at 5,893.
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