There are two settings to look at. One is for "posts per page" in your Blog or maybe Global settings. The other is for number of posts to show in the back office, which is found on your User settings page.

Unless you mean a category name on the front (public) side?

Got linkage where we can see the problem?

EdB wrote:

Unless you mean a category name on the front (public) side?
Got linkage where we can see the problem?

I mean the public side.

Number of posts in a page is setted to 5, and it works when i don't chose a category. But when i click one of them about 50 posts for page are showed, and at the end of the page there are only 2 or 3 pages to browse.

[url=http://skynetxp.110mb.com]Here [/url]is my blog, so you can see what happens when you select one of the categories.

I've just created a new blog, and written some test posts.
When i click a category it works fine. 5 posts per page were showed. Problem is with previous blogs only.

I don't want to think that when it has been converted from 1.10 to 2.4 happened something wrong.

The Category url's were screwed in you original blog link...

They should end with /index.php?cat=27
Your original Cat links ended with /index.php/c27/?blog=7

I've just found the solution.
I took a look at blog settings/SEO. There's an option to tell b2evo how many posts it must shows when a category is picked.
I set it to 5 and now it works.

Okay cool. That explains why half-way through poking around your blog the symptoms changed ;)