Plures/plus - What case should I use with this word?

Hi,

I'm using Rosetta Stone to teach myself Latin. So far, so good. I find this interesting enough to stick with the program and I am enjoying the process, but I just came across sentences that confuse me:

Plures libros habet. - accusative plural
Magister plures libros habet quam discipula. - accusative plural
Plures calamos habet. - accusative plural
Puer plus suci habet quam mater eius. - genitive singular
Vir plus panis habet quam uxor eius. - genitive...

Plures/plus - What case should I use with this word?