Written numbers

[cconiglio]

What is the correct usage when using numbers twenty vs. 20?

[Niall]

In written English, we always tend to write the number out as a word, twenty, especially if we want to be more formal.
With larger numbers, it is best to write the number out in digits, so "129,650" is likely to be written as numbers. Sometimes with large numbers with lots of zeros, it is easier to write the word rather than the figures: so 100,000,000 becomes a hundred million.

[Anglika]

Numbers under 10 should be written in words.

Figures should be used:
(i) for numbers over nine;
(ii) when the material contains a sequence of stated quantities, numbers, ages, etc (example: children in the 7–12 age group);
(iii) for numbers of sections, clauses, paragraphs etc; and
(iv) wherever words would appear clumsy.

However, you might like to look at this for US practice:
The United States Government Printing Office Style Manual 2000