Since you're already running 5 clients, you can see how much VRAM you're currently using with tools like https://www.techpowerup.com/gpuz/
Check the numbers at your current settings, with both a single client and 5 clients, to get an approximate ratio. Then, if you want, you can run a single client at higher settings and check the VRAM usage and extrapolate from there using the ratio you previously determined. It should give you a reasonably accurate rough estimate.