Irrespective of HA or non-HA, RA comes with default setting of 200 Agents per execution server, but again this value is configurable. There is no such benchmarking on number of Execution server ideally it is determined by how many agents you need to have in your Environment topology.
So ideally you can go for high number of agents and in proportion you need to increase the NES's considering the default limit of 200 agents/execution servers
In future road-map we will be supporting higher number of agents per execution server.