Only one asura summon can be summoned per person. The limit does not apply towards the team.
The skill works in combination with Signet of Illusions. With 12 illusion magic you get a level 18 Ice Imp. With 16 illusion magic you get a level 21 Ice Imp which lasts the full recharge of 60 seconds.
The summoned creature:
Does not leave an exploitable corpse when it dies, even though a corpse is visible.
At level 14 (rank 0) it has 10 in Water Magic, while at level 20 (rank 10) it has 15 in Water Magic.