Get count of related entities

I have two entities, Post and Category

A Post belongs to a Category via a entity_reference field type name category_id

I want to get a count of posts belonging to a category (To prevent deleting a category that has posts)

My solution is this:

class PostDP{

  private string entityTypeId = 'entity_post';

  private EntityTypeManagerInterface $em;

  public function __construct(EntityTypeManagerInterface $em)
  {
    $this->em = $em;
  }

  public function getByCategoryId(int $categoryId): array
  {
    $query =  $this->em->getStorage($this->entityTypeId)->getQuery();

    $query->condition('category_id', $categoryId, '=');

    $result = $query->execute();

    $posts = $this->em->getStorage($this->entityTypeId)->loadMultiple($result);

    return $posts;
  }
}

Isn't there another way to get just the count without loading all the posts ?

Actually, I want to start from the Category entity, And just check the count of related posts to a certain category.

Like in this (incomplete) example:

class CategoryDP{

  private string entityTypeId = 'entity_category';

  private EntityTypeManagerInterface $em;

  public function __construct(EntityTypeManagerInterface $em)
  {
    $this->em = $em;
  }

  public function getCountPosts(int $categoryId): array
  {
    $query =  $this->em->getStorage($this->entityTypeId)->getQuery();

    /**
    *@todo Get count posts
    * Build a query that execute the SQL: 
    * 
    * SELECT 
    *   count(*) 
    * FROM 
    *   category c LEFT JOIN post p 
    * ON 
    *   p.category_id = c.id 
    * WHERE 
    *   c.id = :categoryId
    */

    return $count;
  }
}