snow_monkey_related_posts_args

<?php
add_filter(
	'snow_monkey_related_posts_args',
	function( $args ) {
		// 例: 関連記事の表示最大数を8件に変更する
		$args[ 'posts_per_page' ] = 8;

		return $args;
	}
);

<?php
add_filter(
    'snow_monkey_related_posts_args',
    function( $args ) {
        if ( ! isset( $args['post_type'] ) || 'post' !== $args['post_type'] ) {
            return $args;
        }

        $category_ids = wp_get_object_terms( get_the_ID(), 'category', [ 'fields' => 'ids' ] );
        $tax_query[] = [
            'taxonomy' => 'category',
            'field'    => 'term_id',
            'terms'    => $category_ids,
            'operator' => 'IN',
        ];

        $args['tax_query'] = $tax_query;
        return $args;
    }
);