@AlbertoV1 , the rated output power for this inverter is 2000W but let's say in ideal conditions (25C degree and 1000W/m2). Taking into cosnideration these conditions, your inverter should output at its maximum capacity but not always we have "ideal conditions" so the best DC/AC ratio would be 1.33.
Basically, you can put 2.66kW panels on your inverter so you can add another PV panel (5 totally) to be sure that even if it's an overcast day it will output some energy. In fact it's just about the efficiency of the PV system 