This question actually came up in yesterday's new beekeeper session.  The only real answer is "it depends" - largely, it depends on the available forage.  If you have plenty of good nectar and pollen sources, a dozen hives right next to each other are fine.  On the other hand, if you live in a desert where any forage is scarce and spotty, two hives within a mile of each other might be too close to prevent robbing.

In northeast Ohio, our forage is generally pretty good.  Eight colonies in one beeyard would not be unusual.  You might be getting large enough see some drifting, though.  (Drifting is when the bees returning to the apiary get a little confused about which hive they came from.  On average, the hives at the end of a row tend to gain bees while hives in the middle tend to lose slightly.)  You can combat drifting by turning the entrances as you suggested or by adding other landmarks.  Some beekeepers believe that distinctive paint patterns on the front can reduce drifting.

On a purely mechanical note, make sure you leave enough room between your rows not just for yourself but also for whatever cart or trolley you will use to haul out the full honey supers.  Also make sure that you don't accidentally create a restricted path out.  If the bees are having a bad day and you decide that you want to walk away for a while, you don't want to be tripping over your own equipment or bumping into other hives on your way out.