Reworking function for counting custom post type posts count

You have a couple of problems with your code

• customtaxonomy=$mytaxonomy is incorrect. First of all, there is no parameter called customtaxonomy in get_posts. Secondly, your syntaxing is wrong. If you make use of a varaible, your syntaxing should look like this

  'post_type=myposttype&customtaxonomy=' . $mytaxonomy . '&posts_per_page=-1'
  

• You should make use of a proper tax_query to get your posts. You should also make use of get_terms to get all the terms of the selected taxonomy

Here is how your code should look like: (I have also made the post type a variable)

function my_posts_count($taxonomy, $post_type) {
    $terms = get_terms( $taxonomy, 'fields=ids' );

    $args = [
        'post_type' => $post_type,
        'nopaging' => true,
        'tax_query' => [
            [
                'taxonomy' => $taxonomy,
                'field' => 'term_id',
                'terms' => $terms
            ]
        ],
    ];

    $mytaxonomy = get_posts( $args );
    $mytaxonomy_posts_count = count($mytaxonomy);

    echo $mytaxonomy_posts_count;
}

Please note, I have used syntaxing that will not work pre PHP 5.4

You can then use the code as follow

my_posts_count('category', 'post');

EDIT

Rethinking of @birgire comments

I feel we should return the count since we use the get_ prefix, even though WordPress breaks it’s own rule regarding this semi naming convention

I have removed the get_ prefix from your function name. In WordPress, functions which have the get_ prefix returns its output, while functions without the get_ prefix echo its output

The correct way to use your function when using the get_ prefix is as follows

function get_my_posts_count($taxonomy, $post_type) {
    $terms = get_terms( $taxonomy, 'fields=ids' );

    $args = [
        'post_type' => $post_type,
        'nopaging' => true,
        'tax_query' => [
            [
                'taxonomy' => $taxonomy,
                'field' => 'term_id',
                'terms' => $terms
            ]
        ],
    ];

    $mytaxonomy = get_posts( $args );
    $mytaxonomy_posts_count = count($mytaxonomy);

    return $mytaxonomy_posts_count;
}

And in your template call it like this

echo get_my_posts_count('category', 'post');