wp-docs

Edit the file called “YourWordPressFolder/wp-admin/themes.php”

Look for the code below (should start around line 30)
$themes = get_themes();
$ct = current_theme_info();
ksort( $themes );
$theme_total = count( $themes );
$per_page = 15;
Change $per_page from the last row to whatever you wish.

The default value for $per_page is 15.