Optimization on open world maps

Hello community,

I am a new when it comes to UE4 but I have been doing modifications since DOOM2.
I have decided to embark on building my own game however, The Engine is truly a piece of art and is quite beastly so it has me curious about industry standards for getting the most out of open environments while keeping the environment optimized as not to bog down the frame rates.
Specifically, I am starting with foliage, so I do know that utilizing the same asset can cut down on Draw calls improving performance as well as keeping the poly counts down. Has anyone messed around with what a maximum is for say grass assets or have more insight in to this?
I know you could intersect 2 plains and call it good however it’s not as realistic as I would like, I don’t want to accidently box myself into a corner doing a ton of environmental work only to find out that it’s too much and now runs like a slide show.

Any input would be greatly appreciated from the community and a big thanks in advance!