The important rule in sampling is not how many poll respondents are selected but, instead, how they are selected. A reliable sample selects poll respondents randomly or in a manner which insures that everyone in the area being surveyed has a known chance of being selected. Sampling methods and measures of sample reliability or precision are derived from a mathematical science called statistics. Statistics is a subject taught in colleges and some high schools. Text books on the subject are available in most libraries.
Heads is one of only two possible outcomes.
As the number of coin tosses increases, it becomes increasingly more likely to get outcomes which are either very close to half heads or exactly half because, as with two coins, there are more ways to get such outcomes. Sample survey reliability works the same way - but on a much larger scale. As in coin tosses, the most likely sample outcome is the true percentage of whatever it is we are measuring across the total population. Next most likely are outcomes very close to this true percentage. A statement of potential margin of error or sample precision reflects this and often appears in poll stories.