Clarification on RPUs used on indexed queries


I’m reading the Serverless instance costs and it says:

You are charged one RPU for each document read (up to 4KB) or for each index read (up to 256 bytes).

Does “each index read” mean the entire index or each key within the index?

For example, if you had a query that returned 20 documents and only examined 20 documents but it examined 40 keys, would that total 60 RPUs, or would it total 21 RPUs?

Thanks in advance.