Per David-san’s comments, I think you’re likely better off running multiple instances. But with Windows you can be surprised sometimes.
A related point with windows you might not be aware of is that by default it will turn all of your physical CPU cores into two virtual cores. They call this “hyperthreading.” When an application like GP runs, it gets allocated to a core. If you have hyperthreading enabled, it’s really only allocated to half a physical core. But because windows is scheduling hundreds of processes across the available cores (physical or virtual) you really don’t know how enabling or disabling this is going to work out on your machine until you try. The general advice is to leave hyperthreading enabled unless you run into crackling. Then you can try disabling hyperthreading and seeing if it helps.
GP will actually get assigned to two cores from what I can tell. The audio engine will end up on one, and the rest of the program on another. When you open two instances, chances are the audio engines will end up on two different cores. That’s why, per David-san’s experience above, you’ll generally get capacity for more VSTs if you run more instances.
The caveat with that is there’s a “wall” between the instances. If your drums are in one instance and your vocals in another, they have to remain independent. You can’t do clever things like using the vocals to sidechain a drum compressor. At least not without routing the vocal out to your audio interface and then feeding it back in and bringing it into the drum instance. Which would actually probably work fine, but it would be running on a slight delay and (depending on your audio interface) take up an extra input.
Anyway, that’s why my approach is usually “try it out” and if it’s not working out you come back and start talking about different options.
And speaking of options… Depending on your GPU (I use a Radeon 1070GTX) you might get a benefit from uninstalling a bunch of junk they install by default largely for gaming. But if everything works, don’t mess with it.