it's possible to upgrade soldered RAM, but it is an extremely difficult process that can very likely result in the irreversible destruction of your Mac - and we certainly wouldn't recommend it.
21.5in i Mac 2015) so it is impossible to upgrade, and if you tried you would likely damage your Mac.
Without taking a look at your Mac's Activity Monitor at peak usage, it's impossible for us to say how much RAM you really need.
Our advice would be to always bear this in mind when you are buying a new Mac.
There are various build-to-order options when you purchase a new Mac from Apple, and you should get as much RAM as you can afford.