Write the numbers one to ten in full (i.e. “five times out of ten”, not “5 times out of 10”). Use digits for anything higher (“if you’re over 28 years old”, not “if you’re over twenty-eight years old”).
There are a few exceptions to this rule. First, in post titles, always use a digit rather than writing numbers in full (“7 Amazing Craft Projects” not “Seven Amazing Craft Projects”). Another exception would be in situations where an industry standard dictates that a numeral be used for even small numbers, such as in a craft pattern (p1, k2, p2, k1).
For numerals over 999 – insert a comma for clarity. ‘It was over 9,000’. Spell out common fractions, such as one-half, but use a % sign for percentages, ie 50%.
Use ‘500 to 900’ and not ‘500–900’ (except in tables).
Always use "million" (and "billion") in money: $200 million. Use "millions" in phrases: millions of hours.
For ordinal numbers, spell out first to ninth. After that use 10th, 11th...