What is the brand and model of the controller? It matters if it is MPPT or PWM for expanding the array. MPPT has amps limiting so you can "over-panel", but PWM requires some "margin" to stay under its rating when the panel does more than its Isc at times. It will heat up and fry if over-amped for a length of time.
You need the specs of the panel you have before getting more. Info so far on that is not correct as presented. Where did that 6.67amps come from? If you saw that at the battery, then for sure the panel can do more than that. How much more? Three times the mystery amount could be more than the controller can do.