For this mix I've set up a number of Aux mixes. Why? Because it makes much better use of your computer's power to have one instance of a plug-in, with two or more tracks fed to it, compared to using multiple instances across a number of tracks. Also, if you've established a reverb setting, for instance, that's going to be useful on all the vocal tracks, there's no point in repeating those settings on track-after-track.
How you set up Aux mixes in Reaper is pretty well documented, but if anyone wants a walk-through here, I'm happy to oblige.