Numbers
When to spell out numbers is actually a fairly complicated thing, and depends on a host of rules, inclduing the kind of writing you're doing. Books, letters, formal, and informal writng all have a set of "rules" covering numbers.
Generally speaking, numbers should be spelled out in dialogue because it's impossible to say a number. We say words, not numbers.
A number is always spelled out if it begins a sentence. Two numbers expressing different things should not be side by side in a sentence. One should be spelled out. Wrong: "In 1951, 15 states ratified the law." Right: "In 1951, fifteen states ratified the law."
It's actually fine to spell out any number that can be expressed in one or two words, no matter how large it is, as in "The court awarded him two million dollars in punitive damages." But you would write "The court awarded him $2,097,321.50 in punitive damages" because it can't be spelled out in one or two words. Or you could simply write, "The court awarded him more than two million dollars in punitive damages."
Hard and fast rules where numbers are concerned depend on the type of writing, and on in-house rules. In journalism, we were taught "When in doubt, spell it out." But this works well only when you've had some experience.
Best to find a comprehensive guide that defines all the rules.
On and on.