This article explains why you only get one notificiation for multiple friend requests.

To help make managing friend requests easier, Yahoo! Messenger now bundles multiple add requests into one window. This means that if you get more than one friend request, you will only have to click into one notification to see all of your add requests instead of having to click into each add request one at a time.

If the add requests are spam, you can just use the Block All option. You can also tap into each request and have the option to Accept, Decline or Block on an individual basis.