If you want to get it really right, then you need to say:
to one woman: as-salāmu ʻalayki (or ʻalayk)
to two women: as-salāmu ʻalaykumā
to three or more women: as-salāmu ʻalaykunna (or ʻalaykunn)
to a mixed group (men and women): as-salāmu ʻalaykum