loop_shop_per_page

add_filter( 'loop_shop_per_page', 'new_loop_shop_per_page', 99 );

function new_loop_shop_per_page( $cols ) {
    // $cols contains the current number of products per page based on the value stored on Options –> Reading
    // Return the number of products you wanna show per page.
    $cols = 20;
    return $cols;
}