I'm inclined to say that 8 GB is overkill already. I'm perfectly happy with 4 GB.
A thing to note:
Most motherboards perform better with an even number of equal sticks (check the specs of your new laptop at crucial.com). So 4 GB is best done as 2*2, while 8 GB is best done as 2*4. You can't do this with 12 GB. And 16 is surely overkill.
My current laptop is 7 y.o. and VERY sloooow. I'm looking at buying a new laptop and was considering the amount of RAM. I'm debating whether I should customize and buy it with more RAM or buy it standard and add aftermarket RAM. I've been told that for my needs (daily computing, spreadsheets, social media, streaming - no gaming or anything like that) an i5 processor with 8GB would be plenty. My fear/concern is eventually surpassing the 8GB threshold and my comp slowing down again. For that type of usage, is 8GB sufficient? Would 12GB be overkill?