Put enough sponges in so that it makes the humidity at a steady 50-65%. If it goes too high,(75%+) take some water or sponges out. If it goes too low,(below 25%) add some(a very little bit) water. It all depends on what your humidity is.
But you shouldn't have to add too many. The chicks hatching will naturally raise the humidity in the bator. Don't add too much or the water in the egg won't be able to evaporate! You could drown the chick if you have too much humidity.
But I would say keep it at a 50-60% range. No lower than 30% and no higher than 75%.