In copy, spell out numbers from zero to nine; use digits for all numbers 10 and above.
The same rule applies to ordinal numbers: first, second, 11th, 101st
An exception is when the number begins a sentence; then, always spell it out (or reword the sentence): Twenty-five years after their first meeting, the two rivals face off again Saturday.
For numbers above 999,999, spell out million, billion, etc., preceded by the number following the above rules. Use decimals if not a round number: four million; 1.12 million; 49.6 billion
In headlines, digits are acceptable in all cases: Larry Doby, 1st Black AL Player, Dies at 79.
Special rules regarding specific use cases can be found at the links below.
time