The RCs are needed for each transaction. For example (I don't know the actual prices) transferring funds might be 1000 RCs, leaving a comment 2000 RCs, adding a top-level post 3000 RCs. Once spent, these replenish like voting power does - but it means that if there isn't enough RCs, an account can't spam - though it doesn't take much - 50-100 HP is probably enough for most people to do most of what they normally do