First there is almost always a way to process without looping. Loops introduce risk to node-red (a run-away loop will crash node-red) - in simple terms, uninstall the loop node. If you ever need to "loop" an array, use the built in split and join nodes.
Next, there is multiple examples built in to node-red-contrib-opcua .
CTRL-I → examples → node-red-contrib-opcua
In particular OPCUA-MULTI-SUB and OPCUA_READMULTIPLE
I suspect you need to subscribe to multiple nodesids by sending a msg.topic = multiple and the msg.payload as...
[
{ "nodeId": "ns=1;s=NodeIdString1" },
{ "nodeId": "ns=1;s=NodeIdString2" },
{ "nodeId": "ns=1;s=NodeIdString3" }
]
