t.takahashi

Add a UI(select) to change posts displayed on the page.
After installing the plugin, please make the setting from the management screen and write the following code to the theme file you want to display the select boxes.

Posts Per Page Changer UI

Add a UI(select) to change posts displayed on the page.

Chip Bennett

This plugin will allow you to customize the number of posts, orderby, and order parameters for all index page contexts, including the blog posts index, search index, and all archive indexes (category, tag, taxonomy, author, date, archive). Orderby settings support all values relevant to archive index pages, including none, ID, author, title, name, date, modified, rand, and comment_count. Order settings support ASC and DESC.
Note: Plugin settings can be configured via Dashboard -> Settings -> Reading.

cbnet Different Posts Per Page

Customize the number of posts, orderby, and order parameters for all index pages (blog/home, search, category, tag, taxonomy, author, date, and archiv …

Posts Per Page Changer UI vs cbnet Different Posts Per Page