The iPhone maker said on Wednesday that it had also increased its own direct workforce to 90,000 from 80,000 in the last 2.5 years.
READ: Apple sees third quarter profit and revenue beat forecasts, fourth-quarter sales also top expectations
However, the firm did not say how it had arrived at the 2.4mln figure, or how research into the data was funded.
The report followed criticism of the firm from US President Donald Trump, who has accused the company of outsourcing manufacturing jobs to China, where wages are lower.
Apple has hit back at the accusations, saying that in 2018 it spent around US$60bn on services and products from US suppliers, which it plans to increase to US$350bn by 2023.