Mine are laid out in a grid 4' off the walls with 4' spacing between them, but I'm no expert. When/if I build a fixed workbench they may get re-arranged to have light directly over the workbench.
I had 6 originally which worked ok when the door was down but with the door up (blocking the front pair) I wanted more light. With 8 (still only 2 blocked by the door) it's plenty bright.
I've read that a good light level is around 105-110 lumens/sqft and I'm right at 118 with the (8 fixtures * 2 tubes * 2950 lumens) / (20 * 20 ft), so I guess you'd need something like 30 fixtures to get that light level in a 38 x 40 with 2950 lumen tubes (30 * 2 * 2950 = 177000, 38 * 40 = 1520, 177000 / 1520 = 116.45)