Alviria is right. I'll like to add a little more just to clarify.
Even this statement "可能说吗？" is improper. It should 能这样说吗？or 可以这样说吗？
我想多买书：I'd like to buy more books. As Alviria said, I'm rephrasing him, it can also be interpreted as "I'd like to buy books more often." That's perfectly fine too in my opinion.
我想买很多书：I'd like to buy a lot of books. Alviria is exactly right about this too.