The historical Buddha, who was born prince Siddartha Gautama and eventually became known as Shakyamuni (sage of the Sakaya clan) is not considered by Buddhists of all denominations to be the only Buddha in history.
There are, in fact, endless numbers of Buddhas, stretching back to beginningless time, each one appearing at a time where the knowledge of the Dharma has been lost. This re-discovering of the innate nature of the universe is what essentially differentiates Buddhas from their followers, that they rediscover a path that has been previously lost by themselves. They do not create the laws, simply find what is there to be found, and then proceed to teach others.
The Lotus Sutra, which is the best guide to the role of Buddhas, indicates there are three periods of teaching. First, there is the period of teaching the true Dharma, that is teachings which are the genuine ideas and concepts as taught by a Buddha. When these begin to be corrupted and die away, the period of 'counterfeit' Dharma can be said to have started. This Dharma is by no means useless, but it is less than 100% authentic. It can co-exist at the same time as the authentic Dharma.
Eventually all traces of a 'Buddhism' will die away, and Buddhists consider that living beings during this time period will be without a proper guidance. However, once this stage has gone on for some time, another being becomes enlightened to the true nature of reality under their own efforts to understand the world, thus becoming the next Buddha, and starting their preaching career. Beings that undertake the path to becoming a Buddha are called Bodhisattvas, and they are one of the main focus points for Mahayana Buddhism, which emphasises the compassionate aspect of choosing to become enlightened for others rather than for oneself. The desire for enlightenment for others is called the Bodhicitta, and was first experienced by the Buddha in a previous life when he rescued his blind mother from drowning after the merchant boat they were in capsized. Seeing how all beings were essentially lost and suffering like he and his mother lost at sea, he vowed to help save them all. After being eventually reborn during the teaching career of the Buddha Dipankara, he had the good fortune of meeting this Buddha, hearing him teach and was then told he would become a Buddha, as he eventually did in northern India 2500 years ago.
So far, there have been seven Buddhas in the history of the planet Earth:
Buddhist cosmology splits time into period known as Kalpas. A Kalpa is the time period between the creation, destruction and re-creation of a world system. There are 1000 Buddhas born in each Kalpa, each born one after the other, no two being alive at the same time.
The current Kalpa is called the Bhadra, or Virtue, Kalpa, and Shakyamuni Buddha is seen as being the fourth of this present Kalpa. The previous was Adornment Kalpa and the next will be Constellation Kalpa.
Buddhas are not required to be human, but of course they must belong to a kind of being that has a human like level of intelligence. Beings with either too little intelligence, excessively good living conditions or excessively bad living conditions do not have access to the ideal situation required for Buddhahood to be achieved. Good deeds and the intention to become a Buddha by following the Bodhisattva path ensures that a rebirth in an ideal realm eventually occurs.
The Lotus Sutra describes how Buddhas teach using the best means available to them. Since we humans learn using speech, that is the method the Buddha we know of used. Shakyamuni talks of others, though that use other methods most appropriate to their species, including ones who teach by means of sight or even smell.
The next Buddha to appear on Earth will be Maitreya, who currently resides in the Tusita Heavens, awaiting rebirth around 4000 years after the death of Shakyamuni.