What's "too many"? Is there a limit? I'm subscribed to gobs of old threads, and it hasn't been a problem. I've only unsubscribed to a few which remained annoyingly popular but in which I'd lost interest.
I did turn off the email notification feature long ago.
Click user CP (upper nav bar)
The next page should have a list of recent subscriptions near the top. You can click on the link to unsubscribe beneath each thread title.
To see a complete list of your subscriptions, look over to the right of that same section for a link to the complete list.
My "New Subscribed Threads" list is empty. I have accumulated about 280 subscribed threads, most now well hashed. I thought it wise to tidy up the list a bit and have found no way, other than bringing them up one at a time, to reduce the number.