The 'perfect' setup is a lot of single channel receivers. One for every frequency you may ever want to listen to.
You would never miss anything.
Obviously, for most hobbyists that's unreasonable.
Many communications are going on simultaneously and a single scanner can only listen to one at a time.
So, you can try to find an acceptable level of missed communications and balance that with your ability to add more scanners.
The more channels you try to scan, the more you'll miss.
Even a scanner with 100 channels is way too much in my opinion.
In fact, there were times when a 20 channel scanner was too much.
(Aside from the fact that you may not have the proper 'mix' of frequencies for a given event.)
One of my online feeds has around 40 conventional frequencies, which at times is too much. But I've been asked by listeners to include that many.
My private feeds have from 8 to 20 conventional frequencies.
With the ideal being one frequency per radio, and worst case is having too many frequencies, you have to find a point where you are NOT missing most of what's going on.
I have a lot of Uniden scanners and almost every one is maxed out, programming wise.
However, during normal operation, each scanner sits on one agency/city/town/county.
(A few are searching.)
Even though my scanners may hold (and be programmed with) as many as 6,000-25,000 frequencies, it would be silly to expect to scan all of them and get any continuity on any of them. Everything would just be tiny snippets of mostly meaningless nonsense.
My Uniden scanners are like file cabinets full of drawers, folders, and papers. (Systems, Groups, Frequencies)
You can activate any combination of anything in the file cabinet with just a few keystrokes.
I can instantly change the frequencies in all of my scanners without having to reprogram them.
Rich
By the way, this is what it looks and sounds like when you get a Close Call hit on 20+ Uniden scanners at the same time.
That many only because the HT was in the same room. Typically I get CC hits on from 1 to 5 depending on how strong the signal is and how many scanners are stopped on active transmissions.
http://www.youtube.com/watch?v=PSPOaHaf-Qk