10 righteous people (or 9, rounding up) were required to prevent the destruction of humanity. Noach and crew were only 8, therefore they could not save the world. Clearly, by the standard that G-d was using to judge righteousness, no one else in the world was considered righteous.
This number can be found in Rashi, Bereishis 18:32:
אולי ימצאון שם עשרה. עַל הַפָּחוּת לֹא בִקֵּשׁ, אָמַר דוֹר הַמַּבּוּל
הָיוּ ח', נֹחַ וּבָנָיו וּנְשֵׁיהֶם, וְלֹא הִצִּילוּ עַל דּוֹרָם;
וְעַל ט' עַל יְדֵי צֵרוּף כְּבָר בִּקֵּשׁ וְלֹא מָצָא: אולי ימצאון שם
PERADVENTURE THERE SHALL TEN BE FOUND THERE — For a smaller
number he did not plead because he knew already of two instances where
less than ten had failed to save the wicked. He said to himself: In
the generation of the Flood there were eight righteous people, viz.,
Noah, his sons and their wives, and they could not save their
generation (Genesis Rabbah 49:13), and for nine in association with
God he had already pleaded but had found no acceptance.
As crazy as the world is nowadays, I would like to think that there are at least 10 righteous people, and we will never (again) reach the point where there are less than 10 righteous people in the world...
Edit: I sort of meant that last line as a throwaway, but as @JoshK mentioned in the comments, most of this discussion is not practical for future instances, but rather a historical view of the pre-flood generations. However, as is written in Genesis 9:11 (with the explanation of the Sforno), G-d promises that there will never again be a worldwide extinction level event that destroys all of humanity. Therefore, the question of how many righteous people are needed to prevent the destruction of the world nowadays is no longer applicable.