There are currently 5 sets. The base set has 48 units, each other has 45. The upcoming Axis & Allies Miniatures: The War at Sea will not be compatible with the previous set. The current sets are:
Base Set - With a nice coverage of most nations; has a slight Western Front theme to it. Nations include: France, Soviet Union, United Kingdom, United States, Germany, Italy, and Japan. Starter includes 4 two-sided maps.
Set II - Often incorrectly called the Eastern Front expansion; it does have an Eastern Front theme to it, but as with all sets except D-Day the theme only covers around a 1/4 of the set. Adds the minor nations Nationalist China and Poland.
Contested Skies - Apart from obviously introducting aircraft and AA, this set fills out some of the nations a bit more, especially the UK which was sadly neglected in Set II. Adds the minor nations Australia and Poland (No China or Poland).
D-Day - Unlike previous AAM sets this expanion was focued heavily on Operation Overlord. It introduces Heroes and Obstacles, as well as the new minor nation: Canada. Russia only gets 2 units in this set, and Japan only gets 1.
Reserves - This set’s theme is more on the experimental side; including some units that never actually saw battle in WWII. Hungary is introduced as a minor nation in this set, and it builds upon the Canadians, France, Poland, and Italy. The units are fairly nicely spread out between the major nations. Finally it also includes ‘support’ units that can be used by any faction as part of its army (unlike obstacles which are not a part of anyones army).
The New Starter Pack is not much of a starter. It’s two boosters from Base Set/Set II/Contested Skies (two of, not two of each) and two maps.