Good to know
A filter can only be added based on a closed question. So use the field type option option, check box, yes or no or number.
If you have used a quantity field in the registration form, you can choose when setting a filter whether the calculation should take place once or based on the number that a participant enters during registration. For example, if you ask how many people someone is present with, you choose to calculate the filter based on this number in order to correctly calculate the limit.