Getting last thing from config

Hello,
I want to do something like this (I have used reports as an example)

Code (YAML):

Reports:
  1:
    Reporter: MrFishCakes
    Reported: SpigotMC
    Reason: Good community
    Time: 01/05/2017 17:43
  2:
    Reporter: SpigotMC
    Reported: md_5
    Reason: Developer
    Time: 01/05/2017 17:45

But say when a new report was to come in, how would I get the last number (2) so then it makes the new report #3?

Thanks,
MrFishCakes

 

Bumping this thread.