Montón literally means "heap" or "pile," but in this song, it's used in the common idiom "una más del montón," which translates to "just one more of the bunch" or "just another one in the crowd."
This phrase is used to convey that someone is unremarkable or easily replaceable. In the context of the song, the singer is telling his ex that she will just be "one more of the bunch" to her new lovers, implying she won't be special to them, just as she was to him.
Yelsid and Luis Alberto Posada team up for "Una Más Del Montón" [Just One More From The Heap], a song that's all about a breakup where one person is definitely not taking the high road! The singer is calling out their ex for trying to lie and make excuses for their betrayal, saying, "Yo sólo engañas la que tú sabes que mientes y buscas excusas para tu traición" [You only fool the one you know you're lying to and you look for excuses for your betrayal].
The song gets pretty blunt, with the singer telling their ex that they'll regret it and suffer for hurting them. They even say, "Te sientes orgullosa porque dormiste con él en nuestra cama" [You feel proud because you slept with him in our bed] – wow, that's a direct hit! The singer is totally over it, declaring, "Para mí estás muerta en tierra es así" [For me, you're dead in the ground, it's like that] and telling them to forget about them and leave them in peace. The main message is clear: the ex will just be "una más del montón" [just one more from the heap] to anyone new, someone to have fun with for a bit, but nothing more.