Here we discuss three proofs given by Aquinas about unity of God. This is our reference: http://www.newadvent.org/summa/1011.htm
First argument is about the fact that God is simple and this applies only to God. Therefore there is only one God. By simple he means that God is His own nature.
Second argument is about the fact that He is infinitely perfect. Therefore there is only one God. By perfect he means that He exhaust all good attributes.
Third argument is about unity of the creation and the fact that the creation cannot be cause of different God.
My objection to the fist argument: Our minds are also simple yet they are different. Mind being the essence of any being with ability to experience, decide and cause. In fact the definition of mind applies to God too.
My objection to second argument: I can imagine two conscious beings both being perfect.
My objection to third argument: It is not obvious that there is only one universe. There could be many universes which are causally independent.