When writing a novel, there will be numbers used like dates, times, years, ages, money, and just quantities of anything.
Do not start a sentence with a numeral, as it is distracting:
59 tickets were sold last Tuesday.
The common practice is:
Fifty-nine tickets were sold last Tuesday.
Some writers spell out all numbers from one to one hundred and use numerals for anything higher as nine thousand four hundred and sixty-two is a little more wordy than 9462.
“Brian gets £45 an hour,” is somewhat better to read than, “Brian gets fourty-five pounds an hour.”
Conventions on numbers from publisher to publisher varies.
Just stay consistent.