Should be infinite as they have absolutely no size in the z dimension :D
Seriously though Num is right.
The biggest problem with larger robot arrays is that when you run the program at first, it reserves a massive amount of space for the array and this has occasionally been known to crash a program.
By making it a dynamic array, we can bypass this problem because we only need to reserve a small amount of space then dynamically change it throughout the sim.
There may be a potential for running out of memory on some systems at really large numbers of robots but that will be exceeedingly rare.