How you treat numbers is mainly to do with style. A common style is ‘zero to nine, 10 and above’ (or ‘zero to ten, 11 or above’).
Beyond a certain number, though, if the numbers aren’t specific it looks terrible to include them as figures (such as ‘1,000s’). So you might add ‘but hundreds, thousands’ to your style instruction.
You will need to know whether you are including commas or another type of separator in numbers from 1,000 onwards. If it’s up to you to decide, it’s a good idea to include separators, otherwise numbers in the early thousands could be mistaken for years (e.g. 2021).
You will also need to consider how you’re dealing with fractions and high numbers, like millions and trillions, as well as ages, dates, percentages and more. You should…