Ask for the input if it is a reasonable fact the interviewer may be willing to provide. If they do not provide it, state a sensible assumption and continue rather than getting stuck on recall.

The interviewer is usually evaluating how you structure the estimate and work with assumptions, not whether you memorized every country population. Your assumption should be transparent and directionally plausible.

You can also change the approach when another baseline is easier to estimate, such as households, businesses, or customers served by a known channel. The best method is the one you can defend cleanly.

Keep moving and sanity-check the final result. The case interview guide can help with common market-sizing approaches and assumption discipline.

I usually ask once, then make a rounded assumption if they want me to estimate it.