The Empire is evil because it destroys peaceful planets. Aside from that, and the whole dictatorship angle, there’s a sizeable school of thought on The Empire Did Nothing Wrong, which is of course absurd but does make some fair points about galactic stability and making the space trains run on time.
The prequels establish that there are only ever two Sith in existence at any time, at least according to the Sith Rule of Two, a master and an apprentice. This doesn’t mean there aren’t other dark Force wielders out there or even other would-be Sith, but there are only two official Sith in recent galactic history at any one time. Of course, this is an invention of the prequels, and the Original Trilogy was quite vague on the nature of the Sith apart from calling Vader one of their order. Presumably there could be unlimited numbers of Sith.
According to the prequels, the Jedi have a three-tiered system of Padawan(Apprentice), Knight, and Master. A Jedi Knight takes on a Padawan to train as a Jedi Knight. When the Padawan reaches knighthood, my understanding is that their teacher can become an official Jedi Master. This system was not apparent during the Original Trilogy, however, and the working canon by Lucas was that a student or apprentice could become a Jedi Knight if they wished to use a lightsaber and be a warrior monk like Obi-wan, or they could devote themselves to teaching and peace and become a Jedi Master like Yoda. It was not a linear succession, and btw it’s my favored explanation of the Jedi.