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Assuming your not using the '40k in 40 minutes' (sometimes referred to as combat control) rules there is only one necron 500 point list.
20 warriors (two squads of 10)
Lord w/ res. orb
total 500 points exactly.
The Outside is right, here.
In a 500 point game, when not using Combat Patrol rules (which is supposed to be 400 points, anyway), you still must always take the compulsory one HQ and two Troops choices.
Unfortunately for a Necrons army, the minimum squad size of their Warrior squads is ten.
With two squads, you have the following:
Troop Choice 1: 10 Warriors (180 points)
Troop Choice 2: 10 Warriors (180 points)
Which is a total of 360 points. The only HQ choice you can fit in here is a Lord, obvoiusly, which is 100 points.
You have spent a total of 460 points, leaving you 40 points. There is no other unit in the game that you can buy, not even a Wraith (gone over a point), to put in there, so the best thing to do is to upgrade the Lord; and the best upgrade you can give is the Resurrection Orb, which will round your army to exactly 500 points.
A bit boring to play with, but it's still a solid core of gauss and can stand up against anything.
Yeah they pretty much covered it...not much else to say. Just buy a necron lord and your ready to play! (after building your uints of course).
Anyway, good luck!