@loic.wiseflow explained it in this post:
I’ll use Gmail as an example here since I don’t have ClickSend.
You can keep clicking on Add a recipient to add additional emails, but if you don’t know ahead of time how many email you will be sending to, you need to map this field instead.
If you hover over “To” you can see it wants an array:
Click Map to turn that on, then the field will now expect an array as input:
Here one way you could do it, if all the emails were in a comma-separated string
Or, if you have an array aggregator somewhere before the email module, you can just use the array variable from the aggregator. Hope this makes sense.