Hi Lily!
"Elle en a plusieurs" means : "She has several (of them)". The word "en" refers to something that has been said in the previous context (= of them), and allows the speaker to avoid repeating him/herself.
For example :
"Elle adore les chats. Elle en a plusieurs chez elle."
"Des problèmes ? J'en ai plusieurs".
It doesn't matter if the word referred to by "en" is masculine or feminine, "plusieurs" is always spelled this way.
"- Ton ami Pierre, il a une fille, non ?
- Une ? Non, maintenant, il en a plusieurs !"
Hope that helps!!
Good luck :)