To change the default number of seats (or tickets) displayed here:
Go to your organisation's Web Control Panel <YourCountry.patronbase.com/_OrgID/Admin > Web Module Settings > Seats > Scroll down to Default number of seats and enter the desired Default number of seats in the field.
This is a global setting and therefore cannot be changed on a per-production basis.
To change the default number of seats in the dropdown menu here:
You can do that on a per-production basis. See this article here: How many tickets? Please enter how many tickets you would like to book - How to change the default number of seats in the dropdown menu