var pitch = 450;
var width = 400;
var height = 400;
var N = 5;

var bitmap_width = (N-1)*pitch+width;
var bitmap_height = (N-1)*pitch+height;
var doc = app.documents.add(DocumentColorSpace.RGB, bitmap_width, bitmap_height);
var artLayer = doc.layers[0];
var newRGBColor = new RGBColor();
newRGBColor.red =0;
newRGBColor.green = 0;
newRGBColor.blue = 0;
doc.defaultFillColor = newRGBColor;
doc.defaultStroked = false;
app.defaultFilled = true;

	for (Ny=0; Ny<N;Ny++) {
		for (Nx=0;Nx<N;Nx++) {
			artLayer.pathItems.rectangle(Ny*pitch+height, Nx*pitch, width, height);
		}
	}