Crowding loaves in deck oven
I don't know of any "rules of thumb" other than experimenting to find optimal results.  Some loaves will expand a lot more horizontally than others while baking, so it will likely depend on the specific context.

From your description, it sounds like the problem isn't a lack of steam/moisture, but too much.  Crusty breads need steam early on to prohibit crust formation and thus allow maximum open spring.  But then they need space to allow their surfaces to dry out.  If they are almost against each other, moisture that's evaporating from the loaves will linger a bit more and remain somewhat concentrated in the areas between loaves -- thus keeping the crust a bit softer.

As for why loaves added later bake better, it might be a temperature effect.  If you overload an oven (even a commercial one) or have "cool" objects concentrated too close to each other, they'll bring down the effective heat during the early stages of the bake.  In contrast, with breads you often want the strongest possible heat at the beginning of the bake to achieve maximum oven spring before the crust sets.  My guess is that loaves added late take advantage of the fact that the oven has "recovered" in temperature after the massive influx of crowded (cooler) loaves.  

But again, this is just a best guess based on your description.