"Twenty-one" is how you say "第21" in English. The word "twenty" comes from the Old English word "twentig," which means "two ten," and "one" comes from the Old English word "an," which means "one." Here are some example sentences:

I have twenty-one dollars in my wallet. (我钱包里有21美元。)

The party was attended by twenty-one people. (有21个人参加了这个聚会。)

The store is offering a discount of twenty-one percent off all items. (商店正在提供21%的打折优惠。)