The amount of transmission fluid in a 2000 Chevy Silverado with a towing package depends on the transmission type (4L60E, 4L80E, etc.). There's no single answer.
You must consult your owner's manual. The manual will specify the correct capacity for your specific truck's transmission.
Trying to find the capacity online without knowing the exact transmission is unreliable and could lead to incorrect fluid levels, damaging your transmission.