I am trying to find this:

how do we find the country's rank population such as upper class, lower class, and middle class?

Suggestion- The site for the CIA world fact book, can give you that information.

https://www.cia.gov/

To find a country's rank by population classes such as upper class, lower class, and middle class, you would typically need access to reliable and up-to-date socioeconomic data for that country. The availability and methods used to determine population classes may vary from country to country. Here are some general steps you can follow to gather the required information:

1. Identify reliable sources: Look for reputable research institutes, government agencies, or international organizations that conduct studies or publish reports on socioeconomic indicators and population distribution. Examples include the World Bank, International Monetary Fund (IMF), United Nations (UN), and national statistical agencies.

2. Review relevant reports and studies: Explore research papers, studies, and reports that focus on income distribution, wealth disparities, or socioeconomic indicators. Such reports often include data on population breakdown by income brackets or classes.

3. Check official statistics: Visit the website of the country's national statistical agency or central bank, as they usually provide socioeconomic data, including income distribution and wealth inequalities. These data may offer insights into the different population classes.

4. Analyze survey data: Some countries conduct household surveys that provide information about household income and expenditure. These surveys can help identify income or class brackets within the population.

5. Consider indices and rankings: Various indices and rankings exist that gauge income inequality or socio-economic development across countries, such as the Gini Index, Human Development Index (HDI), or the World Wealth Report. While these rankings may not explicitly categorize population classes, they offer valuable insights into the overall distribution of wealth and income.

6. Consult with experts: If you require more specific or nuanced information about population classes in a particular country, consulting experts in the fields of economics, sociology, or demography can be beneficial. They can provide guidance on available data sources or methodologies to determine population classes.

Remember that the categorization of population classes may vary between studies and countries. It's important to consider the specific definitions and methodologies used by the sources you consult to ensure a consistent and accurate understanding of the population distribution across different classes.