How to Specify the Number of Post Revisions in WordPress
By default, WordPress keeps revisions of your posts in two ways, by autosaving changes as you write, and each time you hit “Save Draft”
However, if you would like to reduce, or increase, the number of revisions it keeps, you may use the following steps. To do this, you will need FTP, SFTP, or SSH access to your server.
Step 1) Log in to your server with FTP, SFTP, or SSH
Step 2) Locate and open wp-config.php
Step 3) Add the following line to wp-config.php, changing the number to your preference.
define( 'WP_POST_REVISIONS', 30 );
Step 4) Save wp-config.php