Apparently, there used to be some formal editing rule that said you had to write numbers twice, repeating them once in parentheses, like this:
With the recent amendment of the class definition, the class size increased from three thousand (3,000) to four thousand (4,000).
You still see this number-repeating idea often in legal writing. It's not necessary. The better rule is to spell out numbers one to ten, use numerals for numbers higher than 10, and forget about repeating numbers in parentheses. The revision to the two examples would look like this:
With the recent amendment of the class definition, the class size increased from 3,000 to 4,000.
Much easier to read, isn't it?
Source: Bryan A. Garner, The Winning Brief.