We have had primary nursing on our unit for over 20 years - we all love it! It is not mandatory that each nurse has a primary - but most of us do. We also have what we call "associates" as well. In each room on our unit, we will have either 2-3 assignments, depending on our acuity and our census. For example, if my primary baby is in a room, I might sign on to be an "associate" nurse for the other baby(s) in that same assignment. This way, almost every baby ends up with at least one primary or associate.
As far as how many people are allowed to sign up for each baby, it varies. For the most part, we allow one nurse from each shift (day, night, sometimes pms, weekend nights...). That way each baby has their own "team" of nurses that are consistently taking care of them. We have no problem switching assignments after 2 days if a primary comes in for their shifts - primary nurses get priority over anything else. The only problems we run into are when there are more primary nurses on during a shift than there are assignments in the room. But for the most part, we're pretty good at sharing the assignments. Each of the nurses know the babies in that room, so the babies still get pretty consistant care.
We aren't allowed to start taking associates until you have been off orientation for at least 3 months, that way you still get a variety of experiences. And you can start taking primaries about 6 months after you are finished with orientation.
The parents also enjoy having a consitent team of nurses taking care of their babies. We get to build great relationships w/ the parents - a lot of us continue to keep in touch with our primary baby's families years after discharge. Sometimes though, parents will get a little "concerned" if a nurse who doesn't know their baby is taking care of them. But we just explain that every nurse is qualified to take care of all the babies... it works out just fine!
For the most part, we all love having primaries. One of the downsides, is when you end up w/ a primary who is here for months and months... sometimes you get a little sick of taking care of them. But if that happens, you can always take a few shifts off of taking care of your baby... And sometimes after taking care of a "chronic" for a long time, a lot of us will take a break from having a primary - just to take a breather...
Hope this helps!