It is not given in the case, thus should be based on personal knowledge.
In general, it is a good idea to know the population of the main countries in the world and in your geographic area for market sizing.
It is not necessarily a red flag if you don’t know that, but the interviewer may wonder whether you are apt for a job where such macro numbers are relevant.
I remember one time when I gave a candidate a market sizing question related to India. He began saying the population was 300M – was not a great start ;)