Needle Worm with Coffin Sellar

[Trinity]

i read the rulling on Coffin Sellar and Needle Worm but it doesnt say if needle worm's effect counts as one time or does my opponent take 300 lp damage for every monster that is sent to the gy by nw's effect

 

[Raijinili]

One time, 300 damage. Like Solemn Wishes.

Coffin Seller
Continuous Trap
Each time your opponent's Monster Cards are sent to the Graveyard, inflict 300 points of Direct Damage to your opponent's Life Points.

The relevent text is bolded.