This page is a guide to the spelling of shaman, its plurality, and etymology.
Both shamans and shaman are used as the plural of shaman.
In English, the correct plural form of shaman is shamans. Unlike man or woman, the word sha•man is not derived from the base word man, so the plural form of man cannot be used. The word shaman is actually an incorporated foreign word to the English language (like the French word bureau), and not a word (or derived from a word) from the Middle or Old English like man. Like any incorporated word, it has to follow the same standard English rule for plural forms on nouns (e.g. paper/papers, prima donna/prima donnas) unless it already has a plural form from its original language (like the Latin syllabus/syllabi).
Further, the female form is not shamaness, but rather shamanka.
For more information about the word, please check:
The official Blizzard line on "shaman" vs "shamans", despite the English forms of the word, is that "Both Shaman and Shamans are deemed acceptible (sic) as the plural though we use Shaman."  Blizzard is not always consistent in their usage of it — their use of "shamans" as a plural can be found in various locations.
The term "shaman" is a loan from the Turkic word šamán, the term for such a practitioner, which also gained currency in the wider Turko-Mongol and Tungusic cultures in ancient Siberia. It is unrelated to the Persian word 'Shemen' (meaning idol, idol temple). Evidence is wanting for the plausible suggestion that the Mongolian word is an adoption of Chinese "sha mên", an ordained member of a Buddhist fraternity.
Pronounced either: Sha-man or Shah-man
Sh- (as in she)
-a- (as in take) OR -ah- (as in father)
-m- (as in ham)
-a- (as in about)
-n (as in no)
For a brief introduction to the dialect, please check:
"Shammy" is a derogatory, affectionate, or substitute term of the word shaman (obviously not abbreviated, as it has the same amount of letters). The genesis of the alternate form of the word predates WoW by many years and has been utilized throughout many games. However, with WoW, the term quickly became derogatory or taunting (depending on who was using it: shamans or non-shamans) due to issues with class balance at the game's inception. For many WoW players, the emotions behind the slang form have trailed off as class balances have been worked on. Others have used the alternate form simply for a more playful cast to the class title or to show affection for their character or the class in general.
The correct typing of "shaman" in Spanish is "chamán", pronnounced chaMAN ("cha" as in "choke"). Some Spanish players that use English servers mistake saying "chaman" instead of "shaman" because they think that the correct word in English is just "chamán", but without the Spanish character of "a". No official female form of "chamán" exists.