Multicast Mapping Agent and RP placement
When implementing Multicast PIM sparse mode, it generally doesn’t matter where the RP is placed within such a Multicast topology. The RP is needed only to start new sessions, so the RP experiences little overhead from traffic flow or processing. However, the mapping agent should be placed centrally in order to facilitate RP elections, especially in a hub and spoke topology.
In a smaller network, you could use a single device:
Multicast PIM Auto RP and Mapping Agent single device
If you want to reduce the scope of Auto-RP, you can reduce the TTL: