Use numerals instead of words for numbers.
This is for all numbers including those between 0 and 9. The exception is for numbers above 100s of 1,000s, where you should write the number out in words.
Writing numbers in this way makes it easier for people using screen readers and for people skimming text. Numerals tend to draw the eye and affirm information.
Between ranges of numbers use 'to' rather than a dash.
In numbers over 3 digits use commas to aid understanding.
✔ The session will last 2 to 3 hours.
✔ 1 million
Find out more about how numerals affect reading in this blog from Jakob Nielsen of the Nielsen Norman Group.