Pluralization of “Stachewicz”

Stachewicz looks to me like a Polish name and Polish cz corresponds to English ch as far as I know. ch is a sibilant and words ending in sibilants usually get the -es pluralization. So my vote goes with "Stachewiczes."

I agree. More specifically, though, because of the all the consonants happening within close proximity when the name goes plural. It feels better on the tongue to slip in a bit of vowel. This of course is going more on instinct than anything.

I agree. This is a classic case of spelling it how you say it. I would sound the "e" vowel before the final "s", so go ahead and put it in!

It is definitly stachewiczes. It is pronounced stack-ho-vich.

Whether the final "cz" is pronounced "ch" or "ts", they are both sibilants and thus should be followed by "-es".