An army must always include at least one Core, Special or Rare unit per 500 points played in addition to any Lords and Heroes.

An army just isn't an army unless it has plenty of warriors in its ranks. An army must contain at least one Core, Special or Rare unit for every 500 points played. So, at 750 points you need two units, at 1250 points you need three units, at 2500 points you need 5 units and so on.