Hi @mavl,
Is your cc-multiple selection field of type person or group?
If so, you could use a Select action to only retrieve the e-mail addresses from that array. With an expression you could join these items of the array into a comma separated string value.
Below is an example of that approach.
1. Add a Select Action
In the Select action select your Recipients field (collection). Switch the Map field to text mode. And use the expression below.
concat('"',item()?['Email'],'"')
2. Update your Send an HTTP request to SharePoint action.
In your body use the following expression for the CC field to join the e-mail values into a string value
join(body('Select'), ',')
Below is an example of the whole body value:
{
"properties":
{"__metadata": {"type":"SP.Utilities.EmailProperties"},
"From":"",
"To": {"results":["@{triggerOutputs()?['body/Author/Email']}"]},
"CC": {"results":[@{join(body('Select'), ',')}]},
"Subject":"eTracking slip - @{triggerOutputs()?['body/Title']} ",
"Body":"Details are as follows:<br /><br />"
}
}

This example assumes you have a person or group column setup like below:
