Well theoretically the more ram you have, the denser the object you can work on, but like I said you’re going to hit diminishing returns with hardware. A 15 million poly subtool is a fairly dense subtool, and you’ll never be able to throw enough hardware at it to make working with something like that completely trivial. I can feasibly work on objects with larger polycounts than that, but it’s something I avoid doing for performance/stability reasons.
Far better to develop file discipline, and learn how to keep the per subtool polycount down by eliminating geometry where possible, and splitting off bits into different subtools where possible. The point of 64 bit Zbrush is not to be able to work on a single subtool with unlimited polycount, but to be able to work better with large files consisting of many smaller subtools, each under the practical per subtool limit for a particular machine. If you feel you need a 15 million poly mesh to do what you want, chances are there might be a better way to accomplish what you’re after.
One of the things that makes Zbrush special is that it doesn’t need top end hardware to allow you to to work comfortably with pretty substantial polycounts, assuming an efficient workflow. More powerful and expensive hardware will improve that performance to a degree, but not necessarily to a large enough degree that someone on a budget should feel compelled to bankrupt themselves for. I personally think 16GB of RAM coupled with the best CPU you can afford is perfectly adequate to do great work in Zbrush. You can always add more RAM later pretty easily.