This word is likely a made-up word or a very uncommon word, so it's not a standard part of the English language with a defined plural.
To make it plural, you could:
* Use the regular plural -s: zaxes
* Use the irregular plural -es: zaxen
* Consider context: If "zax" refers to a specific thing, you might be able to use a different word that already has a plural form.
Ultimately, the best way to determine the plural of "zax" would be to rely on the context of how it's being used.