From this tweet by Neil deGrasse Tyson:
The rise of flat-Earthers in the United States...
And from The Guardian
Researchers believe they have identified the prime driver for a startling rise in the number of people who think the Earth is flat...
Is there a rise in the number of people who think the Earth is flat?
I found this YouGov article which states "Just 66% of millennials firmly believe that the earth is round." The same article shows that a higher percentage of respondents believe the Earth is round with an increasing age group. However, YouGov doesn't publicly release its methodology, so I am not particularly inclined to believe its results (especially if uncorroborated).
I have also seen the graph below. However, it indicates (supposedly) search volume, which may not be proportional to the number of flat-Earthers (i.e. if I search "Flat earth," it doesn't necessarily mean I am a flat-Earther).
Note: The tweet mentions "in the United States." This question relates to people in general as in the entire Earth population.