I believe this related to something known as Gauss's Circle Problem....however...I think Gauss was only concerned with the lattice points inside the circle...
The problem actually boils down to finding how many integer pairs (m,n) exist such that:
m^2 + n^2 ≤ r^2
Anyway, using "brute force," there are 35 lattice points above the x axis.....so by symmetry, there are also 35 below
And 11 lattice points lie on the x axis....so we have.....
[ 35 + 35 + 11 ] = 81 {If I counted correctly....!!! }
Note that, if we assume that each square unit of the circle contains one lattice point....then we should have about 25*pi ≈ 79 points....so....81 is pretty close to this.....
P.S. - Maybe heureka or Alan can enlighten us with a presentation of the exact formula for this....I don't know enough higher math to carry it out ....!!!!