If you are under the 1000 tag limit, when you reach it you will get an error message when you try to use your 1001th tag. If you are already over that limit, as I was, you won't get the message but you'll suddenly notice that your tags don't show up for your post if you've used a new one.
I had to open a support ticket to find all of this out when my own tags disappeared. I've been using them from the beginning with keywords designed to help me find a particular post without having to add it into memories. Essentially I was indexing my journal so that in the future I could just think of a likely word that would bring me to the post. It would be great if we had an internal search function for each journal that would do this but we don't. I often store information on LJ and often refer back to previous posts, and this worked really well for me. Apparently it was causing a burden to their servers because a lot of individuals and communities were using tags with great abandon. And why not--no one told us it would be a problem or that we should be frugal in their use!
There is currently no counter that tells you how many you have, so if you are already over you would be reduced to manually counting them. You can copy and paste them into Word for a word count, but if you have tags with more than one word that doesn't work.
I am very frustrated at this sudden reduction in service as a paid customer. I cannot tell you what a pain it's going to be to get back under the limit.
If you have ideas about tags and their functions or improvements to the tag management page, there is a topic under suggestions, here.
A related topic about making our LJs searchable can be found here.