Rayleigh distance of array antenna
The answer is still $$2D^2/\lambda$$ but we need to define D value correctly.
Rayleigh distance of a single horn antenna can be calculated as shown below.
$$R=\dfrac{4A^2}{\lambda}$$
We can define D of array antenna as shown below.

Rayleigh distance of an array antenna can be calculated as shown below.
$$R=\dfrac{2(4A+A)^2}{\lambda}=\dfrac{50A^2}{\lambda}$$