Dunbar's number
Overview
Dunbar's number is a concept proposed in the 1990s by British anthropologist Robin Dunbar. From the correlation between the size of the neocortex in primates and the size of their groups, he estimated the number of social relationships a person can stably maintain at about 150. These 150 people are said to form concentric layers: the innermost layer of 5 (close supporters), then 15 (close friends), 50 (friends), and 150.
Social media and Dunbar's number
Even when your followers on social media number in the thousands, the people you actually interact with regularly rarely go far beyond 150. A follower count measures how well known you are, not how close your relationships are. The people who send you questions in your question box are the group of followers who have shown interest, and they can become the core of your community.
Applying it to a question box
Exchanges in a question box can be seen as moving someone from the outer rings of Dunbar's circles toward the inner ones. The idea is that with their first question they enter the 150 layer, and as the exchanges continue they get closer to the 50 and 15 layers. Rather than chasing numbers, raising the quality of your conversations with a small number of people leads to the growth of your community in the long run.