Because I just figured this out after unsubscribing from over 2,000 sites I wanted to share how to do this.
Going into Manage Subscriptions, selecting all, and then clicking unsubscribe does not work. You get an error “Oops. ..an error occurred. Try again in a few seconds.” which is bad advice because it will always reply with that. It seems the limit is somewhere just below 200 subscriptions.
The trick turned out to be the filter/search box. You can use that to filter to something more manageable, like a 50 or so subscriptions, then selecting them all and unsubscribing.
I didn’t figure out to do this until I was at the M’s. I just typed “M” and the list was truncated to all feeds whose name began with the letter “M”.