Spacing also may be increased as the element count increases. It all depends on what you are trying to accomplish. If you are scanning the array then spacing must usually be decreased with increasing scan angles. Microwave Engineers Handbook (The two black books that have been around for years) have plots of gain, spacing and number of elements that you might find helpful. There is also a plot of spacing, scan angle, and the appearance of grating lobes. I often find that a spacing of around .7 wavelengths as convenient. One other thought. Use a binary number of elements. That keeps the feed network from getting too crazy.