Its ability will trigger for every creature attacking that has a +1/+1 counter on it.
If it said "Whenever one or more creatures you control with a +1/+1 counter on it attacks...", then it would trigger only once no matter how many you had that attacked.
May 13, 2018 2:24 p.m.
Okay that's what I thought! I just like checking just to be sure. I did it a lot for my Sliver EDH deck just because there's so much too it. Thank you for your help Raging_Squiggle! Have a great day! :)