In English, numbers less than how much must be written as words, not as numerals?
It depends on the style of writing. As a rule of thumb, write single-digit numbers as words.
In scientific writing, all numbers are often written as words to be clear about data, etc. However, you will normally see numbers zero through nine written out. In more formal writing, numbers one through ninety-nine are often written out (and sometimes all numbers, regardless of size, although the circumstances when that is applicable rarely arise).
The most important thing to remember is consistency: If you are writing numbers a certain way in a paper, book, or anything else, always write them the same way (1-9 10-99, one to nine 10-99, or whatever).