The number is only there for convenient organisation at the marking centre. This happens all the time. There are people there whose entire job is to account for stray booklets. It's fine.
They don't mark like that. Marking is a synthesis of all the elements of your essay. In reality it's probably one or two /15 but it really depends on the rest of your essay, what level you're writing at, etc.
There's not a specified number. That's not how marking works.
We can't really be sure what impact this would have on your mark because they haven't asked for two RTs in so long that we can't know how this would be included in the marking criteria.
For comparison, if you are asked for one RT in...