Today, it is a simply an issue of style, so it is not wrong to always write numbers with numerals or to always write them in full. For instance, APA recommends that numbers below 10 are written in full, while numbers 10 and above are written with numerals. The Chicago Manual of Style recommends that all numbers below 100 are written in full, with numbers above 100 written as numerals.
Both style guides prohibit mixing spelt out numbers with numbers expressed as numerals, favouring numerals for all numbers when they are mixed.