Plural of 'yes'
Oxford Dictionaries.com gives yeses and yesses as accepted spellings of the plural of yes, whereas searching Cambridge Dictionaries Online for yesses does not return any results.
In the absence of agreement amongst widely accepted authorities, it's a good idea to turn to usage. A quick Google N-gram query for occurrences yeses vs. yesses in their library of books published between 1800 and 2008 suggests that, from the twentieth century onwards, yeses has been the preferred to yesses (the tool is seemingly unable to parse apostrophes); greatly so from around 1950 onwards.
If I were to use yes in its plural form, I would therefore opt for yeses. Given that there is no authoritative consensus, though, it is to a large extent a matter of preference. I imagine there is no fixed, definitive spelling because (anecdotally) it doesn't seem to be a word that is commonly used in the plural, an observation that the N-Gram results appear to support.
Any of the first three. An argument could be made for the fourth. I'd advise that you favour the second, or perhaps the third depending on your dialect. (The OED incidentally offers yes's and yeses, while some other dictionaries offer yesses as well).
The noun yes comes from a mention of the much older adjective yes.
Now, how plural forms apply to mention (as in the use/mention distinction) has been discussed elsewhere here, but two common forms are:
- Use an apostrophe in forming the plural.
- Pluralise it as you would any other word.
(Some other forms include italicising the yes but not the es, and putting the yes part in single or double quotes).
The first is increasingly old-fashioned and sometimes mistaken for "grocer's apostrophe's" where people incorrectly use apostrophes with normal plural forms, but it was once the normal way of pluralising in such contexts (see this answer for more on that).
As such, while historically valid, it's probably best avoided.
Now, from the second we would expect yeses, because that's the normal productive plural form for a word ending in -es.
Meanwhile, doubling the s in this case is rare but not unheard of (c.f. both buses and busses are found, the latter according the the OED being more common in America). Hence yesses also being found.
The last though would be very strange; it half-uses the old form, but in the old form the apostrophe acts as a separator and there is nothing on the other side of the separator.
As such, I'd reject the last, accept the first as valid, but recommend you use either yeses or yesses depending on whether the orthography you normally use has you write buses or busses.
The New Oxford American Dictionary says that the plural of yes is yeses or yesses.