Send email to multiple recipient

You can use MNI for this use case. Please refer below thread to know more about this.

https://community.appian.com/discussions/f/process/12075/repeating-emails-based-on-an-array 

List select the variable in which all your recipients are there and pass that variable in the "to" parameter. Make sure to wrap it in toemailaddress()

If this is about sending the same email to multiple recipients then I think you can add multiple recipients in Send Email smart service, o/w if not, you can use MNI or loop flow 

MNI = Multiple Node Instances

You can find the looping recipes here: [View:https://docs.appian.com/suite/help/23.2/looping.html]

The advantage with looping it should be easier to debug.