You can have as many of them as you can afford and it is something that is worth striving for, because they are useful in many situations. There is no limitation on the number of heroes that you can have. They are ones of the best units in the battlefield, especially at higher levels and, thanks to their abilities, they can boost the rate of development of cities. You should buy heroes always when you can afford it. Then, they are replaced with other heroes. They come in two variations - ones available until they are bought and ones that are available only for a certain number of turns.
You can hire heroes in the market, in the part that becomes available after you invent Rookery (the first era).
