Why can i only have 2400 men in custom battle ?

Because as I can almost get up to 6000 men, I'm guessing that your computer isn't the best available, mine isn't, in fact it's a pile of crap, I wonder if it's possible with a good PC to recreate the battle for Helm's Deep, 10,000 men besieging an army of 300 men.

And there nothing i can do to get more men in battles ?

Better graphics card, video card, processor, I'm not really sure, you'll just have to either upgrade your current PC or get a completely new one, again I'm not an expert, I'm not even competant but if like myself you know someone who knows more about PCs than Ido then you should talk to them as they will be far more helpful than me, even if they don't solve the problem they should be able to do a much better jobof explaining a solution.