There’s some misinformation floating around regarding Lemmy not having a karma system. While many have discovered otherwise, this is for those who may not have.
While it’s not exposed in the Lemmy default user interface, Lemmy does have a fully functional karma system and it is visible in third party clients such as WefWef and Memmy.
Do with that what you will.
Technically, all of them. Your home instance will be the most accurate because it’ll have the most complete data about you but each instance you have federated contact with will end up keeping track. I honestly haven’t pulled apart the code to see how it all comes together but I suspect that may be a reason why they don’t surface the data in the UI, lack of consistency.