There are different conventions for expressing numbers in text.
Remember, the aim is to communicate clearly and make it easy for the reader to understand. It is usually easier to recognise a number when written in numerals (6, 27, 83, 214), than in words (six, twenty-seven, eighty-three, two hundred and fourteen).
I prefer to use numerals, particularly in statistical or other texts where numbers are important and frequent. In general text where numbers may not be so important or are rare, write numbers as words up to nine, then use numerals for 10 and above.
Always use numerals for measurements such as degrees, metres or kilometres, whether the measurement is written as a word (metres) or symbol (m). When a number starts a sentence, write the number in words, or rearrange the sentence.