|What happens when you add too many characters|
As much of a logistical nightmare as that must have been, I can imagine it being a very similar thing when working with multiple characters and plot threads. George R.R. Martin is a good example of this, where he has several characters traipsing all over Westeros and beyond, each with their own fully formed story. Juggling all of these viewpoints and plots, as well as intertwining them successfully, is no mean feat.
I've read a good number of fantasy novels that attempt very much the same thing, but end up being nothing more than a succession of plot points broken up with someone else's name. Characters begin to merge rather than story threads, and at the end you have little more than a flash fiction anthology set in the same world.
I must confess, I've not read any of the Song of Ice and Fire books, but from what I've seen of the TV adaptation and from reviews, Martin manages his characters very well. I have yet to come across an author who works with so many main characters and still maintains coherency in their plot. Based on that, I can only assume that it's damned tough to do. It seems that it's much ore of a safe bet with multiple characters to keep them together and only have one or two offshoots to the main plot (as in Lord of the Rings split the Fellowship into three groups, and only after the first book had firmly established the characters and the story that they were following).
I imagine that a lot of it has to do with the attention span of readers. Not enough characters and you've got a linear, uneventful plot (and if you don't like the character, there's a high chance you'll stop reading soon after picking it up). Multiple characters shake things up and allow us to alternate perspectives a little, but add too many characters and the whole thing becomes too complicated for your average reader to follow. There has to be a balance.
How many characters is too many? Is there a 'golden number' for a story? And do you prefer multiple plot threads or a single main plot?