Ordinal numbers end with the final two letters of the word FirST - 1st SecoND - 2nd ThiRD - 3rd ForTH - 4th FifTH - 5th SixTH - 6th etc

21st is correct. 21th is not. After a number ending in 1 (except 11) you always put st. So 21st, 51st, 4001st and so on. Only 11 takes a th, so we say eleventh. For all numbers ending in 2 (except 12) you put nd. So 2nd, 22nd, 402nd and so on. For 12, you say twelfth. 12th. For numbers ending in 3 (except 13) you put rd. 23rd, 83rd etc. But thirteenth. 13th. For all other numbers, you use th.

No, 21st is the only correct version

21st is the winner 🏆 21th is incorrect 😭 🌈 Nicky