The talmud says that we learn that a nbhi is ten men because the תורה uses the term עדה (community) to refer to the 10 meraglim. One the other hand, סדום was not saved because there were not ten righteous people in the town. Similarly, Noach had to be saved as an individual because (with מתושלת who died just before the flood) there were only 9 people who could be counted. However, the question is raised, why was the halacha derived from the bad circumstance and not the good circumstance.
It appears that the point the Torah is making is that we do not notice the circumstances of things appearing to continue as "normal" but only when there is a major change in the world. Had the meraglim failed to discourage the בני ישראל, they would have entered the land and the entire incident might have been ignored or merely glossed over. Similarly, only when the count for good failed and the punishment brought down do we notice what happened. Thus, we can only see the use of ten as a "community" when the bad community actually causes something to occur or the lack of the ten causes the punishment not to be delayed.
Another point that we see is that while Hashem told Noach to start building the Ark while there were nine people (Noach, his wife, three sons, their wives, and מתושלת), the flood itself only came about after מתושלת died and there were only 8 left. Similarly, Lot only had a theoretical 8 people as well. That is, Lot, his wife, his two unmarried daughters, two son-in-law (from the plural used) and those daughters.. It appears that when it comes to saving people and judging righteousness, Hashem will count himself as part of the עדה, but when it comes the wickedness, the evil people are on their own. This is like the description of free will. If someone insists on "going bad", Hashem will allow him to go on the path that he has chosen. If someone tries to do good, Hashem will help him.