I will list every single time Urmila is mentioned in Ramayana
Janaka offers Urmila to be married to Lakshmana
Now, bull among sages, with great pleasure I give away my two girls, Sita to Rama, bless you, and Urmila to Laksmana—my daughter Sita, who is like a daughter of the gods, whose only bride-price was great strength, and Urmila, my second daughter.
- Sarga 70, Bala Kanda, Valmiki Ramayana, CE
Vishwamitra and Vasistha accept Urmila’s marriage
The Houses of the Iksvakus and the Videhas are beyond conception and beyond measure, bull among men. There is none to equal them. Your majesty, this lawful union of Sita and Urmila with Rama and Laksmana is highly appropriate, the more so because of the perfection of their beauty.
- Sarga 71, Bala Kanda, Valmiki Ramayana, CE
Janaka asks Lakshmana to take the hand of Urmila in the marriage
King Janaka then spoke to Rama, the increaser of Kausalya’s delight: “This daughter of mine, Sita, shall be your lawful wedded wife. Accept her, bless you. Take her hand in yours. Come, Laksmana. Bless you. Accept Urmila, whom I give you. Take her hand without delay.” After speaking in this fashion, Janaka addressed Bharata, saying, “Take Mandavi’s hand in yours, delight of the Raghus.
- Sarga 72, Bala Kanda, Valmiki Ramayana, CE.
Urmila is welcomed to Ayodhya
Kausalya, Sumitra, fair-waisted Kaikeyi, and the king’s other wives busied themselves with receiving the brides. The king’s wives welcomed illustrious Sita, renowned Urmila, and the two daughters of Kusadhvaja. Clad in silken garments and greeted with blessings, they went at once to worship at the shrines of the gods.
- Sarga 76, Bala Kanda, Valmiki Ramayana, CE
Sita mentions Urmila while telling Anasuya about her marriage story
So my father invited my father-in-law, aged King Dasaratha, and afterward bestowed me on the celebrated Rama. And my father, of his own accord, bestowed my younger sister, the lovely and virtuous Urmila, as wife on Laksmana.
- Sarga 110, Ayodhya Kanda, Valmiki Ramayana, CE
This is the last time Urmila is mentioned by name. She never appears or is mentioned by name after this point.
Infact Lakshmana himself never mentions Urmila in the entire Valmiki Ramayana.