One of the primary things I do with my lists is sit down and figure out how many vehicles are possible within a list. For my Imperial Guard I fielded a 26 armoured company at 2500 points for some fun. It was a lot of fun to do so. Was it a strong army list, no. However you learn quite a few things while doing so from list building to on the tabletop, so I did it. Just so you know, it was hard to move around and fit into a deployment zone that had some terrain in it.
Now I come to my Dark Eldar. To figure out what was possible. I did this a long time ago, much closer to when I first got my codex, but it was still a good exercise. I have not fielded it, but I will at some point for fun. Right now I am working hard on a couple list options, doing some fine tuning. Vehicle wise, I am tending to be around 9-12 in my lists for Dark Eldar. So playing around with something like this will come later.
So here it is. There maximum count you can get in a dark eldar army list is 15 vehicles. To do so, you must take two archons in order to get the court of the archon to claim their dedicated transport raiders, and you must take Vect, to get his Dais of Destruction. With that you fill out your elite, troop, and heavy slots with raiders/venoms and Ravagers. Don't forget you must take at least one unit having 9 models in it to ride along with Vect. Decent exercise, not so great of list, but might be fun to play once it is refined a bit.
2500 Pts - Dark Eldar Roster - 15 Vehicle List
Asdrubael Vect @ 440 pts ((C:DE, pg. 55); Unit Type: Infantry; Fleet; Night Vision; Power from Pain; Independent Character; Fearless; Master Tactician; Ancient Nemesis; Ghostplate Armour; Haywire Grenades; Plasma Grenades; Shadow Field; Splinter Pistol; Obsidian Orbs; Scepter of the Dark City)
1 Dais of Destruction (Raider) (Unit Type: Vehicle (Skimmer, Fast, Open-topped); Transport Capacity: 10 models; Dark Lance x3)
Lady Malys @ 130 pts ((C:DE, pg. 53); Unit Type: Infantry; Fleet; Night Vision; Power from Pain; Independent Character; Invulnerable Save (4+); Precognisant; Steel Fan; Plasma Grenades; The Crystal Heart; The Lady's Blade)
(2X) 1 Court of the Archon @ 135 pts ((C:DE, pg. 84); Unit Type: Infantry)
1 Lhamaean (Fleet; Night Vision; Power from Pain; Mistress of Poisons; Kabalite Armour; Splinter Pistol; Poisoned Weapon (2+))
1 Medusae (Fleet; Power from Pain; Eyeburst; Kabalite Armour)
1 Sslyth (Fleet; Feel No Pain; Kabalite Armour; Shardcarbine; Splinter Pistol & CCW)
1 Ur-Ghul (Fleet; Feel No Pain; Furious Charge; Claws and Needle-teeth (CCW))
1 Raider ((C:DE, pg. 91); Unit Type: Vehicle (Skimmer, Fast, Open-topped); Transport Capacity: 10 models; Night Vision; Disintegrator Cannon x1)
(1X) 9 Incubi @ 258 pts ((C:DE, pg. 86); Unit Type: Infantry; Fleet; Night Vision; Power from Pain; Incubus Warsuit; Klaive)
1 Raider ((C:DE, pg. 91); Unit Type: Vehicle (Skimmer, Fast, Open-topped); Transport Capacity: 10 models; Night Vision; Dark Lance x1)
(1X) 3 Incubi @ 126 pts ((C:DE, pg. 86); Unit Type: Infantry; Fleet; Night Vision; Power from Pain; Incubus Warsuit; Klaive)
1 Raider ((C:DE, pg. 91); Unit Type: Vehicle (Skimmer, Fast, Open-topped); Transport Capacity: 10 models; Night Vision; Dark Lance x1)
(1X) 9 Kabalite Trueborn @ 230 pts ((C:DE, pg. 88); Unit Type: Infantry; Fleet; Night Vision; Power from Pain; Splinter Rifle x6; Kabalite Armour; Blaster x3)
1 Dracon (Splinter Rifle)
1 Raider ((C:DE, pg. 91); Unit Type: Vehicle (Skimmer, Fast, Open-topped); Transport Capacity: 10 models; Night Vision; Dark Lance x1)
(2X) 5 Kabalite Warriors @ 120 pts ((C:DE, pg. 89); Unit Type: Infantry; Fleet; Night Vision; Power from Pain; Splinter Rifle x4; Kabalite Armour; Blaster x1)
1 Raider ((C:DE, pg. 91); Unit Type: Vehicle (Skimmer, Fast, Open-topped); Transport Capacity: 10 models; Night Vision; Dark Lance x1)
(2X) 5 Wych Raiders @ 120 pts ((C:DE, pg. 89); Unit Type: Infantry; Fleet; Night Vision; Power from Pain; Dodge (4+); Splinter Pistol & CCW; Combat Drugs; Plasma Grenades; Wychsuit; Haywire Grenades)
1 Raider ((C:DE, pg. 91); Unit Type: Vehicle (Skimmer, Fast, Open-topped); Transport Capacity: 10 models; Night Vision; Dark Lance x1)
(2X) 5 Kabalite Venoms @ 125 pts ((C:DE, pg. 89); Unit Type: Infantry; Fleet; Night Vision; Power from Pain; Splinter Rifle x4; Kabalite Armour; Blaster x1)
1 Venom ((C:DE, pg. 91); Unit Type: Vehicle (Skimmer, Fast, Open-topped); Transport Capacity: 5 models; Night Vision; Flickerfield; Splinter Cannon x2)
(3X) 1 Ravager @ 105 pts ((C:DE, pg. 92); Unit Type: Vehicle (Skimmer, Fast, Open-topped); Night Vision; Aerial Assault; Dark Lance x3)
Total Roster Cost: 2499