How many main named rings does Saturn have?

How to get close to this number

The lettering is the answer. The main rings are labelled with single letters, and if you know the naming convention runs from A through to G you have the count directly. When a scientific catalogue uses letters, the count is the alphabet position of the last one — which is a shortcut worth remembering.

The lettering order is the trap, and it is stated in the explanation: the letters follow the order of discovery rather than the order the rings appear from the planet outward. So reading the sequence as a spatial arrangement produces a completely wrong picture, and this is a common feature of historical scientific naming across astronomy, chemistry and biology.

The deeper point is that "seven rings" is a simplification of something continuous. Each named ring contains countless individual ringlets, with hundreds or more distinct bands visible at close range, so the count describes named groupings rather than physical objects. Whenever a natural phenomenon is counted, check whether the units are natural or imposed by the people doing the counting.