I've read conflicting information on this. Some say it was a global flood while others say it is was a local flood. Science community also leans toward a local flood. It really doesn't matter if it's a local or global flood. What we have to remember is what was the population of the humans and how far spread out were they around that time. Scientists believe this flood occurred around 3000 B.C. so that is a pretty long time ago. It is more than likely all the humans lived around Nuh a.s. area and when the flood came all were drowned except those who went on the ark. So if all the humans were in that region only then a local flood would be enough to drown them all. Scholars call Nuh a.s. the 2nd father of Adam of humanity. It's said the others that came didn't have any kids and humans continued from the offsprings of Nuh a.s.
So it could be local or global, depending which evidence supports more. Others just speculate because the Quran doesn't specifically say one way or another.
We sent Nuh to his people and he remained among them for fifty short of a thousand years; yet the Flood engulfed them while they were wrongdoers. (Qur'an, 29:14)
But they denied him so We rescued him and those with him in the Ark. And We drowned the people who denied Our Signs. They were a blind people. (Qur'an, 7:64)
We revealed to him: "Build the Ship under Our supervision and as We reveal. When Our command comes and water bubbles up from the earth, load into it a pair of every species, and your family-except for those among them against whom the word has already gone ahead. And do not address Me concerning those who do wrong. They shall be drowned." (Qur'an, 23:27)
It was said, "Earth, swallow up your water!" and, "Heaven, hold back your rain!" And the water subsided and the affair was concluded and the Ark came to land on al-Judi. And it was said, "Away with the people of the wrongdoers!" (Qur'an, 11:44)
We sent Nuh to his people: "I am a clear warner to you. Worship none but Allah. I fear for you the punishment of a painful day." (Qur'an, 11:25-26)
But they denied him so We rescued him and those with him in the Ark. And We drowned the people who denied Our Signs. They were a blind people. (Qur'an, 7:64)
So We rescued him and those with him by mercy from Us, and We cut off the last remnant of those who denied Our Signs and were not believers. (Qur'an, 7:72)So We opened the gates of heaven with torrential water and made the earth burst forth with gushing springs. And the waters met together in a way which was decreed. We bore him on a planked and well-caulked ship. (Qur'an, 54:11-13)
Some say based on the top that the verses talk about Nur a.s. so it was localized only. We can see how the scholars interpret these verses and if there's any hadith elaborating them but otherwise regardless of which one it was, it was enough to wipe out all intended humans.