No matter how many slots, there will be a limit on how much RAM you can install. That's down to the motherboard and build of the machine. I've found the best place to look is Crucial (search for it, I have no connection with them other than I have used them successfully). This site does an on-line check of your machine and tells you which RAM options are open to you and will then sell you the bits with full instructions on how to replace them.
I'm only buying machines with 16GB RAM and SSD drives now so they are future-proofed and will give a longer life. It's a bit of overkill but I use them for work and have multiple browsers open simultaneously along with graphics applications and they run at over 8GB regularly in-use.