As far as I know, if you're trying to edit the existing services, while the maximum number of attendees for the existing services is set to 1, it's not available to change the maximum number. This is because the group bookings feature was disabled if you set the maximum number to 1 when you created the service.
In this scenario, you need to create a new service to set the maximum number of attendees. When you add a service, the group bookings feature can be enabled by setting the number to greater than 1. And once the feature is enabled, you can also change the number when you edit the services in the future. As a side note, if you enable the group bookings feature for the service, you'll not be able to set the number to 1 and disable the feature.
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