Learn Italian: Il sedicesimo partecipante è molto stanco. - The sixteenth participant is very tired.

To form ordinal numbers from 11 upwards, you take the standard number, drop its final vowel, and add the suffix -esimo. The number 16 is sedici. We drop the final -i and add -esimo to get sedicesimo.

Does sedicesimo change its ending depending on who is participating?

Yes. Ordinal numbers act like regular adjectives. Because this participant is male, it is il sedicesimo. If the participant were female, it would change to la sedicesima.

The word partecipante ends in -e. How do we know it's masculine here?

Nouns ending in -nte often have the exact same form for both masculine and feminine people. You have to look at the surrounding words to find the gender. Here, the masculine article il, the masculine ending of sedicesimo, and the adjective stanco all confirm this specific participant is male.

Why wouldn't molto change its ending if the participant were female?

In this sentence, molto means 'very' and acts as an adverb describing the adjective stanco (tired). Adverbs never change their endings. If molto meant 'much' or 'a lot of' and described a noun, it would change to match that noun, but as 'very' it is invariable.