I think the only shortcut that came premapped that I use is for commenting out chunks of code: Ctrl+Shift+C
I then have three custom mappings which I use all the time:
F3 - Run highlighted code
F4 - Run entire code script
F5 - Knit .Rmd Files
Edit: I came up with these as I was used to SAS Programming before switching most of my efforts to R & RStudio. In SAS, F3 also runs the highlighted code chunk and so it expanded from there based on habit.