Learn Italian: Questa è la ventesima partita del campionato. - This is the twentieth match of the championship.

To form ordinal numbers from 11 upwards, you drop the final vowel of the normal number and add the suffix -esimo. So, venti drops the -i to become vent-, plus -esimo makes ventesimo. Here it changes its ending to -a to agree with the feminine noun partita.

Why does the sentence start with Questa (feminine) before we even know we are talking about a partita?

In Italian, demonstrative pronouns like questo or questa (this) must agree in gender and number with the noun they refer to, even if that noun comes later in the sentence. Since it refers to the feminine noun partita, we use Questa.

Why do we use del campionato instead of just di campionato for 'of the championship'?

Italian generally requires a definite article before specific nouns like campionato. When the preposition di (of) is followed by the article il (the), they combine to form the articulated preposition del (of the).

Could I use gioco instead of partita to mean 'match'?