Sometimes you want all categories to show on one page. One if the examples is the food menu categories in the feast theme, or the groups category in the Church theme.
To understand the answer to the question, you need to understand the WordPress paging system and how to tweak this to perfection.
Whenever categories pages are shown, WordPress expect multiple posts to be shown, and showing 100 posts on one page is bad practice. For that reason WordPress incorporates a paging system that divides the content into pages.
Mostly the effects of the paging system are a welcome and unnoticeable feature, but it gets a little unnerving when content are spread over more than one page, and you want to add them all on one page.
The answer for this is to set the posts per page in the WordPress theme settings.
To access the solution first click on the settings tab in the WordPress admin menu, and then on reading
second from the top are the settings that will allow you to set the posts per page:
just set the 10 in the example to the number of posts that you want in a page and save the settings.
Now refresh the page for the solution to take effect.