Final answer:
You can add a maximum of four THWN cables to the existing 2'' EMT conduit.
Step-by-step explanation:
In an existing 2'' EMT conduit, you can add THWN cables. THW cables are not recommended for use in EMT conduits.
Based on the information given, if the existing conduit contains three #6 THHN, one #8 THHN, and three THHN cables, you can add a maximum of four THWN cables to the conduit, as long as it does not exceed the fill capacity of the conduit.
It is important to consult the National Electrical Code (NEC) and local electrical regulations to determine the precise fill capacity for the conduit and to ensure compliance with safety standards.