For numbers 1 through 10, spell the numbers. For all numbers above 10, use digits to write the numbers. Always use the digits for numbers when you want the number to stand out or be easily recognized.
Examples:
1. I would like to get ten copies of this proposal.
2. I need to get 25 copies of this proposal.
3. We will send brochures describing our new fund to our top 2500 customers.
4. Follow my 5-step process to write effective email messages.