Seroprevalence studies have been conducted all over the world. For example in Delhi India approximately 50-60% of the population now have antibodies for SARS-CoV-2. There are statistical techniques for adjusting the raw results based on population demographics.

https://www.thehindu.com/news/cities/Delhi/delhi-may-be-head...

Note that seroprevalence studies will underestimate the number of infections because some recovered patients don't produce detectible levels of antibodies.