Hello,
This should obey the rule of English, like if the number is quantity and under 10, it should be written as word, if that bigger than 10, it should be as number. If the number is representing temperature or levels, it should be written as number.
Reference: https://www.scribendi.com/academy/articles/when_to_spell_out_numbers_in_writing.en.html
Regards,
Yutong