There is no limit on the number of people you can tag in a photo. I tested your photo just to be sure and was able to add more people without a problem.
At the end of the list of names you've already tagged there are even more automatically tagged faces with "Click to name" next to them. At the end of this list there is a button to add more faces.
I was using the Family Tree Builder software when I reached this limit.
You caught me on an off day! I assumed you were talking about tagging online.
I just spoke with the developers. There is indeed a limit of 50 associations to a photo.
The head programmer is going to change it soon.
In the meantime you can tag your photos online.