When were counting the number of ring signatures for a certain transaction do we count the number of key images or ring members of each key image?
Monero transactions have a multi-layered ring signature.
This multi-layered ring signature has one or more rings.
There is a ring for every real output being spent. Each of these rings will have the same number of ring members, known as the ring size. These ring members are references to public keys of outputs on the blockchain (sometimes referred to as one-time output public keys or stealth addresses). Ring members which are not the real output being spent are known as decoys. An older term for the number of decoys per ring is "mixin", and therefore mixin = ring size - 1.
For every ring, there is a key image. Therefore the number of key images per transaction is the same as the number of real outputs being spent in the transaction. Each key image anonymously identifies the real output being spent in each ring, and prevents double-spends. There is only one real output being spent per ring.