Over time, you won't need to refer to a guide; you'll be able to assess whether your army is strong enough. This is only sometimes necessary in official tournaments when you need to calculate the required army size to recruit up to 1 peasant. For example, if you have a limited amount of gold and can only buy 2 heroes on a given day, the newly recruited hero (their army and its size, which can vary from 10-4 to 6-0 for a Warlock) appearing under the first recruited hero can be very important.