There are 4 different channels for monitor mixes, so no matter how you slice it, you can have 4 different monitor mixes. That said two different monitors can share the same mix, so you could have a bunch of monitors on the Line 6 link and assign each of them to A, B, C or D. I haven't tried it but I think that if you plugged standard power amps/monitors into the 4 monitor outs, if you then assigned one on the Line 6 Link to Channel A for example it would share the same mix as the monitor that is plugged into Channel A in the monitor out.
This shouldn't be much of a problem though, you should be able to set up some monitor mixes that will be good for more than one musician and have them share that mix. How many years did we go where all musicians shared one monitor mix? In some ways it's better, I personally find it more difficult to play dynamically against the rest of the band if I'm not hearing what they are hearing. It's harder to sing harmony if your mix has mostly your voice and a bit of the other singers etc. I still sometimes do individual mixes but the alterations from one mix to the other are very subtle. Which leads back to the fact that you should be able to do a really decent stage mix with 4 monitor mixes driving x number of monitors. If you're throwing up 6 to 8 monitors on the stage, unless the stage is gigantic you're going to be getting cross-bleed anyway.
I hope this helps a little bit..