The symbol n in statistics generally refers to the number of observations in the data. The capital letter N refers to the number of observations in the entire population whereas the letter n denotes the number of observations in the sample drawn from the population.
Suppose that the population of the city is 10000 people. We draw a sample of size 1000 in order to estimate the average height of people in the city.
Here N = 10000 = Size of the population
and n = 1000 = size of the sample drawn in order to study the population