The plural form is shamans, not shamen; the etymologically-consistent plural form from the original Evenki is shamasal, but this form sees no use in English; the plural form shamans is, however, universally accepted.
↑ 6.06.16.2“shaman” listed in Merriam–Webster’s Online Dictionary (retrieved on 19 September 2008)
↑ 7.07.1“shaman” listed in The American Heritage Dictionary of the English Language, 4th edition (2000)
^ 1978, Carl B. Compton, The Interamerican, volume 25, №3 (Instituto Interamericano, Denton, Texas) We learn from our readers: We have been wrong in writing the word “shamen” as a plural for “shaman”. The word probably comes from Russian and there is no plural except that made by adding an ‘s’ — e.g. Shamans.
^ 2003, Howard Isaac Aronson, Dee Ann Holisky, and Kevin Tuite, Current Trends in Caucasian, East European, and Inner Asian Linguistics — “Dialect Continua in Tungusic: Plural Morphology”, page 103 (John Benjamin’s Publishing Company; →ISBN[…] we note here that -sal tends to exist only as a residual plural marker in -l/-r dialects. For example, in Standard Evenki, as in the Evenki dialects of the Amur basin and the Vivin dialect, use of -sal is limited to a small number of nouns (e.g. bajan “rich person”, pl. bajasal; ɲami:, “female reindeer”, pl. ɲami:sal or ɲami:səl; aβlan “field”, pl. aβlasal; sama:n “shaman”, pl. sama:sal).
^ 2005, Peter Metcalf, Anthropology: The Basics, box 7.3: “Shamanism”, page 132 (Routledge; →ISBN Note that the plural of shaman is shamans, not shamen.