'twenty' is a number, representing the quantity of twenty objects. It can be used as a singular or a plural; as a number it is neither masculine nor feminine. It can also be used for purposes such as ordinal numbers, such as in the phrase 'the twentieth century'.
1. I have twenty dollars left in my wallet.
2. We will be going on a hike on the twentieth birthday of my Grandpa.
3. At the ribbon cutting, Megan said twenty encouraging words.
when writing out 'twenty' as a number, it is usually written in full form and not as its numerical symbol (20). However, if you are following the form of another numerical symbol (e.g. $20) then it is acceptable to use the numerical symbol as well. Also, when writing out large numbers it is important to use commas — for example, twenty should be written as 20, while twenty thousand should be written as 20,000.