Spell out numbers one to ten, but use the numeral for numbers greater than 10.
Ex. I have one pencil, three, erasers, and ten notebooks.
Ex. There are only 18 more days until Christmas.
When using large numbers that extend into the millions and up, use the numeral followed by the amount.
Ex. He made 6 million dollars last year.
Ex. It will cost over 10 billion dollars to get this Mars mission off the ground.