We know these Legions (featured or mentioned in the games):

• http://elderscrolls.wikia.com/wiki/First_Legion (Involved with the Invasion of Akavir)
  
• http://elderscrolls.wikia.com/wiki/Fourth_Legion (Fought in the http://elderscrolls.wikia.com/wiki/Skyrim_Civil_War, commanded by http://elderscrolls.wikia.com/wiki/General_Tullius}
  
• http://elderscrolls.wikia.com/wiki/Fifth_Legion
  
• http://elderscrolls.wikia.com/wiki/Seventh_Legion (Involved with the Invasion of Akavir)
  
• http://elderscrolls.wikia.com/wiki/Eighth_Legion (Fought during the http://elderscrolls.wikia.com/wiki/Great_War)
  
• http://elderscrolls.wikia.com/wiki/Ninth_Legion (Involved with the Invasion of Akavir)
  
• http://elderscrolls.wikia.com/wiki/Tenth_Legion (Involved with the Invasion of Akavir, and the http://elderscrolls.wikia.com/wiki/Great_War)
  
• http://elderscrolls.wikia.com/wiki/Twelth_Legion (Appears in http://elderscrolls.wikia.com/wiki/An_Elder_Scrolls_Novel:_Lord_of_Souls)
  
• http://elderscrolls.wikia.com/wiki/Fourteenth_Legion (Involved with the Invasion of Akavir)
  
• http://elderscrolls.wikia.com/wiki/Seventeenth_Legion (Involved with the Invasion of Akavir)
  
• http://elderscrolls.wikia.com/wiki/Eighteenth_Legion (Mentioned in The Infernal City)

And we know of these Legion ranks:

• Auxiliary
  
• Quaestor
  
• Prefect
  
• Tribune
  
• Legate

And I am assuming the rank after Legate is a General.

Does a General lead a sole Legion? So if there are 18 Legions, then there are 18 Generals, correct? I know some of these Legions have been wiped out though. Under a General, he has Legates, who help lead the Legion, who in turn have Tribunes, and so on and so forth. Who is the higher-authority of the Generals? The Emperor himself? What is the political ruling body of the Empire also? Who rules over the armed forces?

I strongly suggest never ever citing TES Wikia as a source. It's infamous for misinformation, and flat out making things up. Use The Imperial Library or at the very lest UESP for sources.

I see it as this.. An Auxiliary are the lowest ranking members of the Legion. Next, the Quaestor is probably a "Decanus", Or just a commander of ten Soldiers. Then Prefect is probably the "Centurion" Or commander of an entire Century. (100 soldiers.) The Tribune is would probably command an entire Cohort, consisting of roughly 500 men. The Legate would be a commander of an Entire Legion, and a general. (Or Consul if Bethesda wanted to to truly based the Legion's ranking system of of Rome's Legions.) Would be in charge of all Legions within a province. I just can't see the Empire using one Legion to fight of an entire rebellion. I would imagine Tullius commanding atleast 3 Legions.