Why is there only a single parallel universe in Fringe?

I mean according to Walter, there could be any number of infinite universes if I'm not wrong. So why do they only talk about two universes and only one remaining in the end. What about others?

Answer

There can be a number of theories but I think the primary one is because it would have few viewers, certainly not enough to make the show profitable. TV shows are a conveyance for advertising - it is meant to be a profit center, and cannot be just ART. Ideally every television show's advertisers want millions of viewers each week, something that is difficult if the audience is expected to reboot their thinking at every return from a commercial.

"Are we in universe 1 or the alter-1?" is easier to handle than, "Are we in universe 1, alter-1, alter-2, alter-3, or _alter-N?"

The writers would get confused, the actors would miscue which one they were supposedly in, the writers/actors would have to develop/portray different back story and keep them alive (but separate) simultaneously. All this is a lot of trouble, and would develop a diligent core viewership of art followers who, granted would like to keep expanding the universe until it filled their heads. But, honestly, the number of viewers dwindles each time the alter-N is increased because, "It's just too damn hard" to make it, to portray it, and, frankly, for the average viewer to watch it. And every time an audience member leaves, the show's sponsor's pockets gets slightly emptier.

So, they've acknowledged it through dialog that there are theoretically greater than two universes. However, I think they have kept the universes to two to keep the dedicated audience, and to help the actors and the writers.