Does an animosity game require a larger group?
Yes. But to be clear, it's only as adversarial as people make it. They don't have to form cabals and little clubs and work against each other, but the character motivations (especially when egged on by elder Amberties) often put PCs at odds with each other when their goals conflict.
After a few sessions most Amber groups form into two or three little sub-groups with the occassional floater between them. So, yes, it requires a larger group.
Does an animosity game require a potentially world ending plot to keep the pcs even marginally together, role playing?
That is a common and fair criticism of Amber. The problem is power and motivation. With ultimate power, and differing goals, why would these people work together? Family ties help. The rule of not killing family (or not being supposed to) help. But generally you need something big to unify all the PCs periodically or they'll just continue in their separate cabals.
Typically you have the King or Queen or whomever is on the throne pull them together every once and a while and give them a mission by royal drecree, but you tend to lean on "universe-ending threat" to give a sense of stakes. In a game where the end of the world is no big deal, you walk to the next one, it's hard to drive the mission forward over multiple games without a common goal.
//Panjumanju