I think 16gb. I haven’t read/heard anywhere about going higher.
Well, it’s 2019 and I just did this to my old 2012. It was quite easy. Reading comments I realized the main concern regarding disconnecting the battery is to avoid frying the motherboard by accidentally touching areas with something conductive and I agree.
The first re-boot showed only one bay in use and I had to check and found out I didn’t push the top memory card hard enough but once I fix this hiccup and re-booted again I saw the new 16gb and my loving MacBook Pro comes to life so fast!