If you're at the last hundred or so limbs, and you make it to Ch. 14, you can follow visheyry's advice on the Ubermorph area.
If you have beaten Hard Core mode and have the foam finger, then grinding this will be incredibly easy. You can repeatedly shoot enemies after killing them to dismember them even more.
If you play through the whole game, on your second play through you'll probably have over 1000/2500 progress for the achievement, or somewhere around there.
Like many of the other achievements in the game you may check your progress by pressing
and selecting achievements, and then scrolling down to the corresponding achievement.
In this game necromorphs have several limbs you can dismember. Some people have said it was 9 for the regular ones. That may actually be true. If you keep playing the game, you'll encounter many enemies. The achievement's progress will go up pretty fast. From my experience it isn't just the arms, legs and other main things that count. Once the counter reaches 2500/2500, you get this achievement.