It's hard to find a single document for all your questions. There are many technologies to scale network nodes, like DVR, OVN etc. If you have 3 network nodes and you want to add more nodes that will help but again you need to keep in mind about the HA tenant router because it won't do load balancing between multiple network nodes. 

In our case we deployed using a vlan base provider to mitigate networking issues. its all depends on what you are trying to do.