When deciding on the size of a tour group, there are a few factors to consider:

1. Comfort: A larger group can often be uncomfortable for travelers as they may have less personal space and may have to wait longer for activities such as restroom breaks or meals. Additionally, larger groups can make it harder for tour guides to manage and provide personalized attention to each traveler.
2. Cost: Smaller groups may be more expensive per person, but they often offer a more intimate and personalized experience.
3. Logistics: The size of the group may affect the types of activities that can be planned, as some activities may have limits on the number of people that can participate.
4. Sustainability: A smaller group can have less of an impact on the environment and local communities.

Overall, it's important to find a balance between the size of the group, the activities planned, and the comfort of the travelers. Depending on the specific tour and destination, a group size of around 10-20 people can offer a good balance between these factors.