Bingowings said:
Building more Death Stars makes sense....
Two Death Stars would have been better but a whole cluster of the buggers would have been better still....
A number of Death Stars is almost a requirement for a galactic empire. A single Death Star, in the hands of an ambitious leader, would have been enough to usurp Palpatine's rule. Several Death Stars, in the hands of rivals, provides some stability for the Emperor. Even if his rule weakened over time, the multiplicity of Death Stars would guarantee mutual assured destruction should the factions ever engage in open civil warfare, so the status quo would remain mostly intact.
Technical means might have given the Emperor sufficient control over a single Death Star to mitigate the threat of an ambitious commander. The superlaser could have been made dependent on emergency action messages from Coruscant, cryptographically secure codes stored in Palpatine's "nuclear football," so it could never be used without his permission. Or the superlaser could have been designed to fail after a single discharge, ensuring the potential usurper would only get to use it once before facing the inevitable counterstrike. (You come at the Emperor, you best not miss.) This would also have forced the Empire to be more deliberative and circumspect in choosing its targets, so the idea probably would have been a nonstarter for the trigger-happy Tarkin.
The Death Star II doesn't need a defense. The lack of Death Stars III-X does. Why did they stop at two? Given the speed and secrecy with which the Death Stars were constructed (or one of them, if you consider the prequels canon), it cannot have been for lack of construction capacity. Was Palpatine limited to only what he could do in secret? Was he concerned that open construction of Death Stars would have inflamed public sentiment against him? Did public opinion turn against him and superweaponry after Alderaan? Endless speculation is possible, but endless writing is not, so I'll stop here.