As of my knowledge cutoff date of September 2021, India was the second most populous country in the world, with a population of over 1.3 billion people. The most populous country in the world was China, with a population of over 1.4 billion people. However, population figures are constantly changing due to factors such as birth rates, mortality rates, migration, and other demographic changes, so it's possible that these rankings may have shifted since then.
It's worth noting that population size alone does not necessarily indicate the strength or success of a country, and that there are many other factors that contribute to a country's development and prosperity, such as economic stability, education, healthcare, and political stability.