For NGOs: To know how much material to bring (e.g., giveaways, if allowed), ask the festival in advance how many visitors are expected. Calculate some extras, and in case some things are left over, use them at another event.
For festivals: Make sure to talk to your project partners about the number of tickets you have sold and about how much staff and other related people will be present on site; also, clearly indicate if you would like some materials to be used/distributed by your own staff in order to not limit the project to one spot in the festival site.
For NGOs & festivals: Please take into consideration your project values and guidelines; any material produced and distributed for the project should be
inclusive and
sustainable.
SocialTables explains how to calculate capacities in their article “The Trick to Calculate Event Capacity Correctly in 3 Simple Steps”:
https://www.socialtables.com/blog/event-planning/capacity-party-space-calculator/