"Once per turn" triggers

Board state: I have on board [[Blech, Loafing Pest]], two Pests, and [[Hollowmurk Siege]] (having chosen Sultai).

Initiating event: I gain 1 life, triggering Blech to put a counter on himself and each of the two Pests.

Question: how many cards do I draw from Hollowmurk Siege?