By default WordPress does not allow you to limit how many taxonomy terms can be attached to a post.
Enabling this plugin allows the use of the “limit” attribute when initially calling register_taxonomy().
If the limit is set to 1 then only one term can be added to the post. If the limit is greater than one then that many
terms may be added to the post. If the limit is attribute is not set, or it’s set to -1 this plugin has no effect.
Example code showing the use of the limit attribute:
register_taxonomy('artists', 'art_work', [
'label' => 'Artists',
'show_ui' => true,
'limit' => 1,
]);
This example shows the use of a custom taxonomy called “artists” and the post type is “art_work”.
Here is another example but this time it’s editing the previously registered taxonomy “post_tag”:
// First load the taxonomy
$taxonomy = get_taxonomy('post_tag');
// Edit it to only allow 2 tags to be added per post
$taxonomy->limit = 2;
// Re-save it with our updated settings (note casting the object to an array before saving)
register_taxonomy('post_tag', 'post', (array)$taxonomy);