The rpcthreads parameter is the number of independent API requests that can be processed in parallel. In reality however there are many locks inside the product that means you won't see much performance benefit with a value above 2.
The rpckeepalive setting means that MultiChain waits for the client to close connections to the API, rather than closing them itself. It is an independent setting but can in practice reduce the effective number of rpcthreads, if clients don't close their connections properly.
