Uttara

King Virata was very happy to have won the war against Susharma and Duryodhana. He came to know that the five Brahmins who had helped him protect his kingdom were none other than the brave Pandavas. He was very grateful. He also came to know that for one year Arjuna had lived in palace as a servant where he taught music to his daughter, Uttara. He was overwhelmed with gratitude and said, "Brave Arjuna, please accept my daughter's hand in marriage." Arjuna replied, "I have been your daughter's teacher, I am like her father, I can't marry her. Instead of me ,marry her to my brave son, Abhimanyu."

King Virata's joy had no bounds. Arjuna called for his son who was at that time in Dwarka. The wedding was celebrated with great splendour. Abhimanyu's mother, Subhadra; Abhimanyu's uncles, Krishna and Balarama; also graced the occasion

Vatsala

How many wife's can a 16 year old have?
According to me Abhimanyu had only one wife. Uttara the princess of Matsya.

Abhimanyu is supposed to be very young when he died in the war, and was married for hardly less than a year.

All the other upapandavas were unmarried. Abhimanyu was a little child when Pandavas went on exile and all upapandavas were preoccupied with training themselves to avenge the humiliation of their fathers and as soon as exile was over within a year war happened.

Abhimanyu married Uttara to cement the alliance of Pandavas with Matsya for the war

There are versions which mention his marriage to Balram's daughter, but it is highly unlikely, because if Abhimanyu became Balram's son-in-law then Balram would have fought with Pandavas, as a father of the bride rules of the time made it mandatory for him to fight on the side of his son in law (family)

The story of Abhimanyu's marriage to Vatsala is not found in the Mahabharata or the Puranas but is popular in folk narrative, especially in Andhra Pradesh (where it was portrayed by the film Mayabazaar).

Vatsala was a daughter of Balarama.
Balarama had great affection for Duryodana. Before the birth of Abhimanyu, he wanted his sister Subhadra to marry Duryodana, instead of Arjuna. Hence, Krishna, who was aware of this, saw to it that Arjuna abducted Subhadra and they got married. In the case of Abhimanyu, that same scenario repeated a generation later.

Lakshmana was the son of Duryodana, and Balarama wanted his daughter Vatsala to marry Lakshmana instead of Abhimanyu. Hence, Krishna advised Abhimanyu to seek help from Ghatothkacha to solve this problem. Ghatothkacha abducted Vatsala thereby enabling Abhimanyu to eventually marry Vatsala.