While more subjects may be desirable, there is no “magic number” that’s correct, and ten may be enough to get useful results. It is not necessarily a problem that the blocks in the mountain were placed later, as long as this is taken into account in the analysis and he compares them against time after their placement. What is a problem, however, is that temperature is not the only variable that differs between the two experimental groups. For one thing, the air pressure will be different at the higher mountain altitude than it will be in the meadow. There may be other differences between the two areas that are not accounted for as well, such as wind and weather patterns.