You should be able to add up to 400 items to your Watch List, or up to 200 items to other custom lists if you are using a desktop browser.
If you are at the limit, try deleting some old or ended items to make more space for new items.
If you are not near that limit but you are having problems adding new items, try signing out of eBay, restarting your browser and then signing back in. That can often help with Watch List issues.
