Working on a recent project in collaboration with Keith Devon, we needed a way in which to limit the number of tags which could be applied to a post – one of the clients requirements. Therefore we set about a way to do this and produced the WP Limit Tags WordPress plugin.
The plugin, on the WordPress repository for download from your WordPress admin, creates a small settings screen. Here you can set the number of tags that are allowed and tick the post types you wish the functionality to be active on.
It is worth noting that at the moment the functionality works on any taxonomy which is non-hierarchical, such as post tags. Maybe in the future it would be good to be able to add different settings for the different post types and taxonomies.
Below is a short video which shows what the plugin does: