I think this does not work for a somewhat simple reason. If you put a limit in the auth rule you might be actually blocking data you want to get.
Consider your example above. The auth rule also controls the getTodo. So if you always limit it to two you will only ever be able to read 0x4c6cd2ed and 0x4c6cd2ee. So if you tried to do getToDo(id: "0x4c6cd2ef") { id } you would be blocked.
So the solution is not this in an auth rule, but a new feature to put a max limit somewhere for a generated query*