How many Buddhas are there?

Buddhas are born once in a Kalpa. Altogether there are 28 buddhas.

Kalpa is a time period which is described as when deva(similar angel) flys among the skys once a hundred years when her clothe brush against himalayas a little of himalayas will fall. like that one day it will come to the ground level. That period is know as kalpa.

There is no famous in famous in Buddhism. Every buddha's story is given as example in buddha's preaching. Maybe you have heard of few. Because Every bodhisatwa(one who travel's in the path of buddha) have to get niyatha wiwarana(prediction from buddha).
In that case on one got prediction from buddha to be Maytree in next kalpa.

No he is not living in this world now. Maybe there are one bodhisatwa.
Once he got predictions he will live in thawsitha diwya(like a heaven) waiting for his time.

It will vary according from school to school, in Theravada tradition there are 6 Buddhas, 4 before Shakyamuni Buddha, Himself and Maytrea (Next Buddha). This is what Buddha saw when he contemplate this world and past lifes while meditating.

In Tibetan Buddhism they talk about 35 Buddhas, Chinese Buddhism will probably give you a different number and so on, but answering strictly from a Suttas' perspective I believe 6 is the answer, at least that is what Shakyamuni was able to see we cannot know if Maytrea will look into the future and see more Buddhas.

Bear in mind I am talking about full enlighted Buddhas that teach the Dhamma, not private Buddhas, most of Buddhas are private Buddhas and they can go on unnoticed by regular people.

In one Buddha Sasana period there can be only one Buddha. Buddhas are indeed few if you look at the gap between Buddhas. But time into the past is infinite hence there are infinite number of Buddhas. Our Buddha got definite prophecy from 28 Buddhas Dipankara Buddha.

There are many people that are technically buddhas in this world right now (having removed the four veils, as described in "the luminous mind: the way of the buddha" by Kalu Rinpoche), however, according to the buddhist traditions, there are "founder" buddhas only from time to time. Each "founder" buddha guides many beings to buddhahood, that in turn will guide many beings to buddhahood. By many I don't mean millions, but I guess, without proof, that there are several thousand buddhas right now in this world.

According to the Mahayana text the Lotus Sutra, the number of Buddhas are innumerable. In fact a significant proportion of the text is spent emphasising just how many Buddhas (and Bodhisattvas) there are and just how many world systems they cover.

This is just from the introduction

and the nun Yasodharâ, the mother of Râhula, along with her train; (further) with eighty thousand Bodhisattvas, all unable to slide back, endowed with the spells of supreme, perfect enlightenment, firmly standing in wisdom; who moved onward the never deviating wheel of the law; who had propitiated many hundred thousands of Buddhas; who under many hundred thousands of Buddhas had planted the roots of goodness, had been intimate with many hundred thousands of Buddhas, were in body and mind fully penetrated with the feeling of charity

The text goes on for another twenty eight chapters drilling that message home.

NOTE - even though it is highly repetitive I love this text. If you want a flavour of the cosmic vastness of Mahayana Buddhism then I recommend. It's a mind blowing read.

There had been infinite number of Buddhas before and there will be many more.

In this Kalpa, We already have four Buddhas and there will be one more Buddha. That's why this Kalpa is called Bhadda-kappa.

Extracted it from here.

In the current kalpa, it is said we are fortunate to have experienced five Buddhas. Kusanda Buddha, Konagamana Buddha, Kasyapsa Buddha and Buddha Shakyamuni. The Buddha of the future is called Maitreya Buddha.

Read more about Bhadda-kappa from here. At Page 12