Please go to Theme Directory > inc > customizer.php on line no: 1795 you will find below line of code.
'input_attrs' => array( 'min' => 4, 'max' => 50, 'style' => 'width: 50px;' ),
replace the desire number on value of ‘max’ like mentioned below.
'max' => 55,
NOTE: More number of post you will show on a page will affects the Load time for page. We always recommend to show less number of post as possible and also recommend to customize the Theme code only after making the Child Theme so that your customized code wont get lost when updating the Theme to New Version.
Thanks