My last name has two occurrences of the letter "s" in it, so in speech I tell people all the time that it's spelled "with two esses". However I don't know how to express such a thing in writing. I can see half decent reasons for any of "s's", "S's", "ses", "Ses", "ss", "Ss", or "esses" . Which is/are correct?
You can use any of the following:
with two esses
with two Ss
with two S's
with two s's
Ability to format permitting, and font suitably different between italic and roman, you could also italicise the first S to signify mention over use.
with two S s
(Note extra space above due to how the formatting here works, not intended).
All are unclear in a few different ways, and apostrophes in plurals are becoming less and less common - pluralising a letter like this is probably the one case that is surviving the best as the cases it was once acceptable die out (see this for more on that)., but still not as clearly correct as it once was.
I would both avoid the problem of pluralisation and add extra precision with:
With a double-S.