The Summon Mastery skill improves the strength of what you summon, but does not allow you to have more than one summon at the same time. You can have your necromancer creature summoned along with a demon or ghost, though.
IIRC the video preview for Summon Mastery in the skill window does show more than one summoned creature, but that is not an accurate reflection of the skill (the previews may show more than one instance of an effect, and ignore the cooldown time, etc, in order to demonstrate the skill in just a few seconds).