Staff have been introducing “infinite scroll" to themes — a site wide change that will be applied to all themes that are suitable for the implementation. http://en.forums.wordpress.com/topic/to-infinity?replies=137
Disabling" the infinite scroll feature to get rid of the pop-up in the footer results in a "Load More Posts" tab, and you can't disable that "Load More Posts" tab from appearing and have "previous post" and "next post" links, without the Custom Design upgrade. http://en.support.wordpress.com/custom-design/
Is there any way to set a different number for Home and Archive?
No there isn't. Wecannot edit the Archives or the Categories pages which are automatically generated by the software when you publish posts. Whether or not full posts are displayed on Categories Pages and Archives pages is theme dependent ie. it's coded into the template and we cannot edit templates or themes. http://en.support.wordpress.com/themes/editing-themes/
In the majority of themes, posts on category or other archive pages show up just as they do on the main posts page in the blog. (See this page > Settings > Reading and note how many posts you choose there sets it for all). But in some themes only the beginnings of posts are shown, and in a few themes you only get titles or thumbnails. See here for a theme list of themes that may display the way you want > Category/archive pages: full posts or not http://wpbtips.wordpress.com/2009/03/25/full-posts-in-archive-pages/