firstly - The galaxy Ultron controls have 5 trillion stars in it roughly so it is a stary galaxy which also increases the danger for the people living in the galaxy cause if their local star died out, becoming blackhole a lot of people are going to die like from this but this also allows Ultron to access the energy from the stars and have a very good power supply. Ultron controls over 100,000,000,000,000 or over 100 trillion planets, all of the galaxy. Here's the thing many of the planets taken over by his race had no life on them so they were turned into forge worlds and resource worlds for mining their ores. A lot of the planets had intelligent organic life on them, to take over them Ultron would spend a maximum of a decade and a minimum of on the planet studying the people, their government, language, culture, everything. Then he would go back to his home prepare an army and come offer the people to join his empire, he would speak in their language and offer to help, just making himself more likable to the government and people. However, if he finds the government corrupt and how bad the people are on the planet, he would send a squad of cobras to take over the government and make reforms then take over the planet. His army you can say is so vast that each world has a proper army to protect it never falling short since the robots cover the majority of the army and the organic beings are only one division casualties are very small when the army goes to war.