Sein Sieg machte aus ihm einen Helden.

English Translation

His victory made him a hero.

Einen Helden sounds plural to me, but the English translation is singular—warum?

As far as I understand - der Held is an N-noun:

Masculine and neuter n-nouns take -(e)n for genitive, dative and accusative

where an N-noun is

A masculine or neuter noun with genitive singular and nominative plural ending in -(e)n is called an n-noun or weak noun (German: schwaches Substantiv ).

And in this sentence einen Helden is the accusative case.

Check out the declension chart here, Held - Wiktionary.