In my opinon the fewer monitors the better. First, how much benifit do you really think stereo wedges will give a vocalist? Think about what they usually ask for: 1) More me. 2) More of another vocalist, usually the lead if they are not the lead. 3) More of an instrument they are playing if this applies. 4) occasionally more of a single instrument and it is usually something they are using to sing to like a keyboard or accoustic guitar or a track. Being on stage one usually hears most of the instruments from the proximity to them. What they are striving for is to hear themselves. The more you put in a wedge the less clarity you will have. You will find the more experienced singers will only want themselves or occasionally themselves and the lead vocal in the wedge. It is interesting to note that they will also usually want the lead as loud or even a littler louder so they can blend with it.
In ears are providing isolation so you can actually build a true stereo mix if you want to. They are completely different from listening to wedges. They even can have their own unique problems like dealing with the latency of digital processing causing a comb filter effect with the sound arriving to ones ears through the solid parts of the body from the vocal chords, but that is another topic.
I have done thousands of shows and what I teach all my engineers to do with monitors is to put only the mic in front of the wedge in as a place to start. 98% of the time that is all that is needed except the occasionals I mentioned above. Another thing that usually helps is to cut the lows on the wedge below 100hz and even as high as 125-200hz on some singers if you have the capability and you are not putting anything in the wedge like kik drum or a track that has something like that in it. This will go a long way in clearing up the things you don't want in there like stage rumble, plosives, mic handling noise, etc.
I promise you most people really don't know what they actually need in the wedge. If you set up the monitors like I recommend above and tell the group to run through a song and see what they need, if anything at all, you will usually seem like the professional (which you are) in their eyes. If there is a seperate monitor console at the stage or you can see and communitate with everyone you can make most of the adjustments as you are doing that first song run through. I usually point to each one and then their monitor and give the OK sign to see if they need anything. At the end of the run through I always ask, "Are the monitors good?" and, "Does anybody need anything else?" It is amazing how once most people feel confident with you looking out for their monitor needs they quickly turn their attention to their performance and enjoying the show.