'Twenty,' when referring to a quantity, signifies everything, or fullness, because it is ten twice. In Genesis 18:31, 'twenty', like all numbers occurring in the Word, signifies things and states. The significance of 'twenty' can be seen in its factors, namely, ten and two.

In the Word, 'Ten,' and 'tenths,' signify remains, i.e. every good and truth which the Lord puts into a person from infancy to the end of life.

'Ten twice,' 'two tenths,' and 'twenty' signify the same thing, but in a higher degree, namely, good. Good things in triplicate signify remains, namely, the good of infancy, the good of ignorance, and the good of intelligence.

(参考: Arcana Coelestia 10222)