When writing numbers at the beginning of a sentence, always write the words out in full.

So, “Twenty sailors were on deck at the time” is correct.

Within a sentence, numbers can be written in words – “There were twenty sailors on deck” – or in numerals – “There were 20 sailors on deck.”

There are different styles, so the use of words or figures in text can vary. If in doubt, the most common rule is that single digits (one, two, three, etc) are written in words and the number 10 and greater are written in figures.

“Of the 18 children in class, only five were in yellow house.”