Do a query something like this
$args = array(
'post_type' => 'your-post-type',
'post_per_page' => -1, // get them all
'post_status' => 'publish', // or whatever status your looking for
'meta_query' => array(
array(
'key' => 'your-field-name'
'compare' => 'EXISTS',
)
)
)
$query = new WP_Query($args);
$post_count = count($query->posts);