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See also NUMBERS.

Form plural with s, no apostrophe.

In general use figures for the following:

cardinal and ordinal numbers larger than nine, ninth
ages of persons, animals, and things above nine
calibers: .50-caliber machine gun, 40-mm cannon
centuries: the 1800s
Egyptian dynasties: 1st dynasty, 12th dynasty
fractions (See FRACTIONS.)
lens apertures: f/16
latitude and longitude: 72° 54' 23" N, 165° 53' W
page numbers
percentages; but one percent
sessions of Congress above the Ninth
sums of money that are not whole units or that are above units of nine:
            $1.21, $11 (not $11.00); but eight dollars. (See MONEY.)
times of day with a.m. and p.m.

Use Roman numerals for rulers, popes, Olympiads, personal names, ships, and a few other special uses. For formation, see Number Table in Webster's.
Olympics: XIII Olympiad