"She has a lot of rings and she wants another one."
Translation:Elle a beaucoup de bagues et elle en veut une autre.
"Plein de" is the familiar version of "beaucoup de", so it is an invariable prepositional phrase: "Elle a plein de bagues".
Yet, if you describe a box that is full of rings, you will say "Elle est pleine de bagues". The reason is that in this case "plein(e)(s)" is an adjective.