So. Many. Dragons.

I did some math and such over the last few days.

Using the basic 15 dragons (metallics, chromatics, and gems), plus the 5 templates that can actually apply to those dragons (celestials, half-celestial, fiendish, half-fiend, and - believe it or not - half-dragon) you can have 3,675 different dragons. This ignores the undead templates that can apply to dragons (ghost, skeleton, zombie).

This includes ones that have the same templates but, because of the order, end with a technically different dragon. For example: There is no difference between a Red-dragon/fiendish/Half-amethyst dragon/celestial/half-celestial and a Red-dragon/fiendish/half-amethyst/Half-celestial/celestial. However, there is a difference between a Amethyst/half-celestial/half-topaz/fiendish and a Amethyst/fiendish/half-topaz/half-celestial. The first is an evil dragon, the second is a good outsider.

What's odd about the whole thing is that the most powerful versions of any dragon variety (except gem dragons) will be of opposite alignment than the base version (and an outsider). So your basic gold dragon will be LG, but the Gold/celestial/half-celestial/half-red/fiendish/half-fiend will be evil and have a CR 12 points higher. Gem dragons, normally neutral will have both a good and evil version of this level of power.

If you go with just the monster manual, and remove the gem dragons (which hurts me, I love gem dragons), you cut the number down to 1,140. The huge difference is the gem dragons neutrality which makes them more versatile as far as what templates can be applied to them (specifically the celestial/fiendish and half-celestial/half-fiend templates).

If you add the skeleton and zombie templates, the number climbs to 11,025 because every single one of the 3,675 can take on either of the two templates. Adding the ghost template is more complicated. It can apply to any of the dragons, but none of the outsiders. That means it can apply to only 1,885 of them. This brings the grant total to 12,910. If you are going to include the gem dragons from MM2, you could also include the spellstitched template which would only apply to the zombies or skeletons, doubling their number and bringing the total to 20,260

So there are actually 1,885 "dragons", another 1,790 heavily draconic outsiders, and 16,585 draconic undead.

Keeping in mind, this doesn't account for the 12 different age categories for each dragon, for that, multiple the whole thing by 12. (243,120, almost a quarter million).