How many people left Omelas out of guilt and how many survived?

We are not told exactly how many people leave Omelas. We do know that they leave because they cannot tolerate the knowledge that their happiness rests on the misery of another person. However, because the story says that they leave one by one, the implication is that not many go. We are told that:

At times one of the adolescent girls or boys who go to see the child does not go home to weep or rage, does not, in fact, go home at all. Sometimes also a man or woman much older falls silent for a day or two, and then leaves home.

Note that the text says "one of" the teenagers or "a[n]" older person leaves. The suggestion is that most survive the shock of seeing the miserable child living in filth. Most people have the ability to rationalize the situation. The story tells us that often these people decide that, because their happiness carries with it the knowledge of suffering, it is not a "vapid, irresponsible" happiness. Because they know someone else is suffering for them, they are more compassionate and kinder to their own children. Also, we can surmise that most people survive their guilt and stay, because the city is thronged with crowds for festivals. If most people left, the city would be emptied out.
Le Guin wrote the story to condemn the idea of "the greatest happiness for the greatest number." She does not approve of the many basing their happiness on the misery of other people, even if it is just one other person.