To configure the node to run only once (no matter the number of items), open the node, go to settings, and turn on the option
Execute Once. Check the image below.
Does the API Authentication node depend on data from the Google Sheets node? If not, you can do something as shown below. Remember that the
Add Items node has to reference only the first output of the
API Authentication node. Else, it will work only for the first item. You do this by adding $items(0) before the expression. All together should look similar to
$items(0).$node["HTTP Request"]["variable that holds the key"]. This is just needed for the authentication part. When referencing the output of the
Google Sheets node, you can simply do
$node["Google Sheets"]["key you need"].