I have been able to link 5 reservations except in one instance. That instance is when one party used a different agent. I don't know if the class of booking matters too, since we have always been in the same class.
Here is what really disappoints me. Right now we have 8 people in 4 separate reservations linked. If we want to make, and pay for, a specialty dining reservation it wont use each individuals on file credit card or OBC. One person has to pay for all unless I sign on under each persons account and make each reservation separately and make four reservations for 2. I did this for an upcoming cruise for dinner on the Magic Carpet. I figured I would link when I board.
The other issue that disappoints me is not being able to bid on cabin upgrades with another couple we travel with. If only one of us were to win our bid to upgrade to Agua or Suite, we couldn't dine together, so we never bother trying.