If you’re unable to add more friends it could be for a few reasons:
1) You’ve already reached the max of 100 friends
2) The person you’re trying to add has already reached their 100 friend max.
If you know for certain you have well under the 100 friend limit just move onto the next name on your list to add. If you’re adding someone from a posting on an EA Forum or other site they often get maxed out quickly. If you’re finding you can’t add a lot of names on your list try adding your name to the Tapped-Out Origin ID page, and others will add you.