gcolyer,
You were right to doubt your initial example!
If your glass contained two million molecules when it was full of air then it would simply end up with one million once you half filled it with water. Unless you changed something else (such as the pressure), all that you've done is halve the enclosing volume and halve the amout of air which leaves the density exactly the same.
Your statement
Would you then agree that in that pint glass you have maximum air density? You should agree as you cannot get more than one pint of air in a pint glass.
is a bit misleading. The term 'one pint of air' is meaningless on its own - you could easily get more air molecules into the glass by raising the air pressure. Lowering the glass from the table to the floor or taking it down a flight of stairs would do just that!