Was Buddha a single human man? Or is Buddha a concept or maybe a spirit often reborn?
"Buddha" is a title, not a name. It means "awake" or "enlightened". Usually, when one mentions "the Buddha", one is referring to Gautama Sidharta, who lived about 2600 years ago and founded what is now called Buddhism.
Some traditions, however, say that he was just one in a long line of Buddhas, who come into the world whenever the dharma is forgotten by the people.
Yes, Buddha was a one human. There were previous humans who attained the same knowledge and understanding before.
Gautama was the recent Buddha's name. He was born between c. 563 BCE to 483 BCE ( roughly) and lived for 80 years.
History of Buddhas has a brief summary of the event when Gautama Buddha talked about previous Buddhas.
Venerable Særiputta asks the Buddha when it was that he first resolved to work for attainment of the Buddhahood and what pæramis (virtues towards perfection) he had fulfilled to achieve his goal of Perfect Enlightenment
Then Buddha described:
How as Sumedha the hermit, being inspired by Deepankara Buddha, he makes the resolution for the attainment of Buddhahood and how the Buddha Deepankara gives the hermit Sumedha his blessing prophesying that Sumedha would become a Buddha by the name of Gotama after a lapse of four asankya and a hundred thousand kappas (world cycles).
This could be answered either or both ways, yes and/or no.
No -- Religious heroes are established in aggrandizement proportionate to their distance in time and forfeit from recorded, accurate history. Those featuring extraordinary and outrageous, supernatural characters display the most obvious departures from reliable records, and at best they are embellishments on numerous legendary and partly-historical human beings.
The Buddha, so called, is precisely of this character, enshrouded in legend, having for hundreds of years been described orally before accreted and proposed biographic and tutelary details were integrated to this shifting picture. Much more was not known about any human being behind the title than was known, and politics and social trends, fashions, and fads all contributed to the assemblage which comes down to us as an inspirational and stellar success story.
Yes -- Gautama Buddha, also known as Sakyamuni and Siddhartha, was one individual human being described as having special birth signs, foretold as the World Monarch or the World Teacher, enshrouded in protection away from instances of sickness, old age, death, and the holy life so as to give him no incentive to abandon his princely and worldly station.
Instead he is said to have, as a young adult, discovered these phenomena, abandoned his wife and family, taken up austerities with the spiritual authorities of India in his time period, sufficiently mastered them so as to gain insight into their limitations, and perfected the Middle Way. His 8-Fold Path or Marga is said capable of yielding the worthy result of Nirvana, the extinguishment of craving and the anguish it is said to cause.
While others may have been provided titular resonance insofar as they also are supposed to have 'awaken' (and are thus called 'buddhas'), only one individual is known particularly for this feat of wakeful discovery or for his transcendance of death in Parinirvana thereafter.