My rules:
1.) 1 track = 1 coaster
Thus, Lightning Racer and Gwazi are 2 credits, but Kennywood's Racer is 1.
2.) If it's on RCDB.com, it counts.
Thus, powered coasters and liquid coasters like Journey to Atlantis count.
3.) Carnival and fair coaster count, but they are counted once per fair operator.
Thus, I could ride 20 Wacky Worms and get credit for them all as long as they are from different fair operators.
There's not real hard and fast rule to count coasters. But you make the rules and be consistent.