The simplest guide is that numbers below 10 are written, and those 10 and above are in numerals.
But there are many exceptions, for
- Category (measurements, dates, very large numbers, etc.),
- Place in the sentence/in proximity to other numbers (aka, within "eyeshot"),
- Specific style needs (AP and APA use the single-digit scheme, above, CMOS has numbers written up to ninety-nine, and MLA has a hybrid set of specifications) -- For most writing, especially business/marketing, we think AP is a smart way to go.