Depending on the kind of content you post and the theme you chose, Tumblr's default setting for the number of posts on your blog's main page may not work for you. If your posts are fairly short, for example, a sidebar with a lot of information may end up longer than your blog's content, making it necessary for you to increase the number of displayed posts. If you post mainly photographs, on the other hand, you could have each post on its own page to make it stand out more.
Changing the Number
You can edit the number of posts that display on your main page from the Settings menu. Click the "Settings" button on your Tumblr dashboard and then click the name of the blog for which you want to adjust the number of posts displayed. Click "Customize" and scroll to the bottom of the theme settings, and then click "Advanced." Select the number of posts using the "Posts per Page" drop-down menu. Click "Save" to save your changes, followed by the arrow, and then click "Exit" to return to the dashboard. You can have a maximum of 15 posts per page; if your blog uses infinite scrolling, the number you choose is how many posts will be shown when the page first loads, with more appearing as the reader reaches the bottom of the page.