It's hard to say since different people play different ways, and with different people. High Elf Silver Helms are famously nasty, especially en masse and with a Ld10 General- so that may mean you want to take a lot of them, or maybe you want to be more reasonable and only keep 6-8. Similarly High Elf shooting is quite good (though that varies depending on whether you want to take the 'tournament' list or regular one)- you may want to stock up on that, but that can also be seen as a lousy thing to do.
Personally, I'd make sure to keep the minimum requirements of a 3,500pt army, then make a range of 2000pt armies, going to various extremes that I think I would like to play with. Total all units in all 2000pt lists, ignoring overlap, and you'll probably end up with quite a cut-down army that still leaves many options- and will be fieldable at 3,500pts.