There are twenty students in this class.
二十 (twenty) and 二重 (double, twofold) are pronounced identically as にじゅう but have completely unrelated meanings, so listeners must rely on context.
三十 means thirty while 二十 means twenty; the two look alike as tens-numbers (Ｘ十) and are easy to misread quickly.
二十歳 (はたち) is the special reading used for a person's age of twenty, while 二十 (にじゅう) is the ordinary reading for the number twenty in all other contexts.
Standard kanji spelling; the most common written form in vertical text and formal contexts.
A single-character kanji for twenty, occasionally seen in traditional contexts or proper nouns.
Arabic numeral spelling, very common in horizontal writing and for dates, prices, etc.
Formal/legal kanji spelling (daiji) used to prevent forgery on documents like checks.
Mixed kanji-zero spelling, sometimes used in vertical writing or for stylistic effect.
Archaic variant of 廿; rarely encountered in modern Japanese.
Compound of 二 (ni, 'two') and 十 (jū, 'ten'). The obsolete reading はた is a native Japanese word for twenty.