Thank you very much for the answer, now it works, but I noticed one bug. In the home page, after set to show 6 items per page, if I hit the second page button, the home page reloads but nothing changes, still showing the first portfolio page. Check it up on my website.
Also, I was wondering if I could set the shortcode to show only 6 items like [portfolio columns=3 showposts=6 orderby=title], or something like that, so as I only restrict showing 6 items when I use the sortcode and not on the portfolio page.
Thank you in advance.