Does "1 lime leaf" mean a pair of leaves, or half a pair?
What does "1 leaf" mean?

Since there does not seem to be a clear consensus, this answer will attempt to weigh both positions.

For the sake of clarity, in this answer:


Digits (1, 2, etc.) refer to the number indicated in the ingredient list;
"Full leaf" means two attached segments, as they grow from the tree;
"Half leaf" means one segment (attached or detached).


Evidence-based arguments

This section presents sources that address the meaning of "1 leaf" either explicitly or implicitly. The most explicit sources so far tend to fall into the "half leaf" camp.

1 leaf means "full"


fourwindsgrowers.com:


  The leaves are double-lobed and the top leaflets are slightly pointed.
  The leaflet attached below is broadened on its upper edge. The size of
  the leaves can vary quite a bit, from less than an inch to several
  inches long. The medium to large size, more mature leaves are darker
  in color and usually preferred for cooking.


This excerpt is attributed on the above website to a book titled It Rains Fishes: Legends, Traditions and the Joys of Thai Cooking by Kasma Loha-unchit (Pomegranate Artbooks, 1995). See also: thaifoodandtravel.com (below, in the "unclear" section).


1 leaf means "half"


foodsubs.com:


  [The leaves] look as if two glossy, dark green leaves were
  joined together end to end, forming a figure-eight pattern.  Most Thai
  recipes count each double leaf as two separate leaves.

mvcitrus.org.au:


  The leaves ... are a dark green color with a
  glossy sheen. They come in two parts: the top leaflet is lightly
  pointed at its tip and is attached to another leaflet beneath that is
  broader on its upper edge. The size of the leaves can vary quite a
  bit, from less than an inch to several inches long. ... In recipes
  that call for them, estimate the number to use according to their
  size, with the average single leaflet (detached from its double) of
  about two inches long and an inch wide equaling one leaf.



Unclear


thaitable.com:


  A leaf has two connecting leaves which I call ‘double leaves’.


Sounds like this one is in both camps at the same time. Each leaf is two leaves—very zen.
importfood.com:


  lime leaves grow in doubles. Sizes vary, but the average
  individual leaf is approx 2" long.


This site indicates that there are ~25-30 individual fresh leaves per dry ounce. May need to contact the seller and/or take independent measurements to determine confidently which camp this falls into.
cooksinfo.com:


  The leaves are a dark, glossy green and look like two leaves stuck
  together tip to tip; they are often described as being "double-lobed"
  or "double-leaves." Each separate leaf is about 2 inches long (5 cm.)


This could go either way; "[they] look like two" implies "but are actually one," yet the phrase "separate leaf" implies the opposite. Length cited agrees with above, but this could just mean they have a common source (or one is taken from the other).
thaifoodandtravel.com:


  The dark green leaves come in double form – a more pointed top leaf
  joined to a more rounded bottom leaf.


The zen approach again. Note: This is written by the author of the book mentioned in the Four Winds Growers entry, above.
theepicentre.com:


  They come in two parts: the top leaflet is lightly
  pointed at its tip and is attached to another leaflet beneath that is
  broader on its upper edge. The size of the leaves can vary quite a
  bit, from less than an inch to several inches long.

specialtyproduce.com:


  The leaves grow in pairs from stem to stem. Sizes can vary from leaf
  to leaf, though they each have a teaspoon shape with pointed ends that
  can also be sharp, certainly a natural defense mechanism.


I was inclined at first to put this in the "half" camp based on "the leaves grow in pairs." However, "they each have a teaspoon shape with pointed ends" gives me pause because the "half" at the stem is typically rounded or cardioid in shape. So the "each" there sounds like it refers to each "full" leaf. The "half" at the stem has no sharp tip, being attached at either end to something else.
boxedupchef.com:

No quote, but interesting to note that this is the first source I've come across that shows an image of the leaves separated into two "halves." Even the sources above that are explicitly in the "half" camp show images of the conjoined "full" leaf.


Practical arguments

This section discusses why (and when) one or the other interpretation might be more appropriate, practically speaking.

1 leaf means "full"


"Half leaves" do not occur by themselves; only "full leaves" grow on the tree, so each one/single leaf must be this "full leaf."
"Full leaves" vary less in size than "half leaves" because they always contain one "half" from the branch side (usually smaller) and one "half" from the tip side (usually larger).
Using "full leaves" when cooking imparts a more intense flavor to the dish, which may be particularly important when using frozen or dried leaves (avoids under-seasoning the dish if you read the recipe incorrectly).
Thinking in terms of "full leaves" when writing a recipe ensures that at worst, the reader will use less of the ingredient than is called for, which may do less harm than using more than is called for.
In some trees, especially very young trees, the "half" on the branch side is so small as to be almost entirely stem; it would not make sense to refer to this "half" as "1 leaf" since it has no practical use.


1 leaf means "half"


Each "half" is distinct, shaped more like a typical leaf than the "full" pair, and easily separated from its neighbor.
The most explicit sources give "half" as the common meaning of "1 leaf," so using this meaning may be the quickest way to be understood most of the time by people who are already familiar with the ingredient.
Using "half leaves" when cooking imparts a less intense flavor to the dish, which may be particularly important when using fresh leaves (avoids potentially overpowering the dish if you read the recipe incorrectly).


A better terminology?

I would encourage anyone who shares or writes a recipe calling for lime leaves to clarify exactly how to measure "1 leaf" or to avoid using the term "leaf" at all. Weight measure would be ideal in many cases but can be impractical, depending on the rest of the ingredients in the recipe (and how much time you care to devote to measurement). Some alternatives include:


This recipe uses "stem" instead of "leaf," which is more precise in that two "half leaves" and one "full leaf" both share a single stem.
This recipe refers to both "leaves" and "pairs" to make the quantity explicit.