A possible solution for the 50x50 grid is [(0,34) , (48,1)].
A possible solution for the 100x100 grid is [(62, 0), (64, 0)].
The "**" part was phrased wrong, since it omitted the possibility that two bots always suffice, for any grid size. Many solvers found solutions with two bots for large grid sizes, the largest is [(133, 974), (1099, 421)] for N=1106 found by Alper Halbutogullari. Finding a proof that two bots always suffice seems to be an interesting problem.