Why is the plural form of piano "pianos" and not "pianoes"?
Plural of nouns ending in -o:
Nouns ending in -o can add either -s or -es in the plural, and some can be spelled either way.
As a general rule, most nouns ending in -o add -s to make the plural:
So you have:
solo solos, zero zeros, avocado avocados
Note that:
Those which have a vowel before the final -o always just add -s:
like:
studio studios, zoo zoos, embryo embryos.
So:
piano, pianos
Oxford Dictionary