When I was reading the manga it showed that both Susanoos have 4 arms, and since I saw Madara's Susanoo with 4 arms I thought it was because of EMS, so am I wrong or could anybody explain me?
Itachi's susanoo had 4 arms when he used it vs Sasuke... One arm held the sealing pot and on the same limb another arm held the sealing sword. The left side of Itachi's Susanoo holds the shield.
Itachis susanoo also had 2 hands on the arm with the totsuka sword so i think you can choose how many arms you want but it requires alot of chakra for more parts.