He's part of an organisation that have tried to wipe out the Jedi order in thousands of years. To maintain proper treason levels within this organisation, they've decided to use the rule of two for about 1000 years.
Be one dude, with one apprentice, disband the main governing body and you'll need quite a big army to control your new empire.
There's also that tiny point of that you don't need to be a grand ruler to ensure that there's warnings about the threat and cause a military build up. In particular when being a master manipulator. It's the equivalent of justifying a spy master to become dictator because he predicted a war.
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