My Potato is a Potato.

[MiRai]

Im currently multiboxing 9 accounts on Eve Online (running low specs)
I get some spikes of low fps every now and then, but im used to it now so im not bothered too much about that problem.
I am really worried about if it would survive if i expand my setup to 12(or higher).

You've pretty much answered this yourself...

Currently with 9 running with just some chromestuffs/testing streaming and whatnots, CPU shifts between 60% up to 90%
My 16gb ram is barely keeping up with this, depending on what i have up other onto of all clients its running at 13-15gb

I don't know how you'd be able to expand if you're already sitting at 90% in some areas.

I dont know how to check how the gpu is manageing so that might also be a whole new bottleneck...

I'd use GPU-Z or HWiNFO for that.

Any advice on what i could change fairly cheap or perhaps its just best to let the potato die?

Well, you don't have to let it die as it's meeting your current need, but you can't just upgrade one thing on the machine and expect it to be good since you have multiple bottlenecks. You can increase the RAM, but you can't just swap out the CPU without changing out the motherboard, and if you go with the latest and greatest, then you can't re-use your RAM. We also don't know if the GPU is sitting at a high load or not.