Correct spelling, explanation: ordinal numbers are usually created by adding “th” to the end of the cardinal number. Sometimes other letters in the number name are changed due to pronunciation, but generally adding this suffix is the universal rule. The exceptions are 1, 2, and 3 (ordinal numbers are: 1st – first, 2nd – second, 3rd – third), which end with st, nd and rd also when are the endings of a bigger numbers (for example 22nd, 43rd, 191st). Number 20 doesn’t end with any of these, so the suffix for the ordinal form is -th. Therefore 20th is the correct form, and 20nd doesn’t exist.

Definition of 20th:
numeral, the ordinal number between 19th and 21st
My aunt is crazy about cats – this one is her 20th.
I want to fly a balloon by my 20th birthday.

Incorrect spelling, explanation: 20nd is the wrong form because cardinal number 20 doesn’t end with 2, so the ordinal number suffix is -th, as for most numbers. The correct form, thus, is 20th. If the number ends with 1, 2, or 3, the endings are respectively -st, -nd, and -rd. Suffix for any other ordinal number, not ending with these three, is -th.

Incorrect spelling

Incorrect spelling