The two Death Stars that we see in the Star Wars films weren't operational for very long, having met their ends rather quickly at the hands of the Rebels.
Before the Rebels threw a monkey wrench into the Empire's plans, how many simultaneous Death Stars were envisioned by the Empire?
If more than one, then how large would the eventual fleet of Death Stars have been, and how large was the volume of space that was intended to be patrolled by each one?
Extended universe information is fine.
According to Wookieepedia (backed up as well by my recollection of reading nearly everything EU had on the period between 0ABY and Vong):
One Death Star was completely built by the Empire, with a second and third one never reaching full completion, as well as a prototype being in existence. In addition, a scaled-down version, the Tarkin, and a Hutt knockoff, the Darksaber, would be created by the Empire and Durga the Hutt, respectively.
There's also a very popular fan theory that even DS2 wasn't meant to be "for realz" - and merely a convenient, albeit super-expensive, "it's a TRAP" for the Rebellion from the get-go.
As far as "how many were planned", no canon information exists. But since Tarkin - as a Grand Moff - basically "appropriated" one, we can make an informed guess that - given unlimited resourcing - one would be created for each of the oversectors controlled by a Grand Moff.
I haven't found canon reference for this yet, but Wookieepedia mentions that there were 20 of those initially...
Under the Sector Governance Decree, twenty of these new Moffs were also given responsibility for supporting the Republic's twenty Sector Armies, and their military territories, generally matching the sphere of influence of their respective Sector Armies, were dubbed oversectors.
...but later the oversectors and Grand Moff assignments changed and there's no firm canon info on what the final # was.
Interestingly, the main thrust of your question (how many were envisioned) was moot for the Emperor, who didn't view the Death Star as much a military vehicle as a symbol - and thus even one would have been enough for his purposes.
This is elaborated on in James Luceno's New Disney Canon novel Tarkin:
... As powerful as the battle station might become, its real purpose was to serve as a tangible symbol and constant reminder of the power of the dark side, and to free Sidious from having to portray that part.
In the game Star Wars Rebellion it is possible to create multiple death stars. Which at least points to it being in the realm of possibility that multiple simultaneous death stars were envisaged.
