From what I have observed with my tanks, your tank will find its balance between whatever food they find and number of snails. They eat any food, not only snails. I was actually told to feed my fish sparingly while I was trying to get the assassins to deal with my pond snail problem because if other foods are available to the assassins they may not need to go hunting for the other snails.
I had an explosion of assassins in my tank too, sometime after they had finished off the last pond snails. I see a lot less of my assassins now, but it seems the number has been steady for months (somewhere between 15 and 20 I had never taken the time to get an accurate number).