It would take 1,200,000 mosquitoes, each sucking once, to completely drain the average human of blood.

The calculation is inaccurate. An average human has about 5 liters of blood, and mosquitoes consume approximately 0.001 to 0.01 mL per feeding. It would require somewhere in the range of hundreds of thousands to potentially millions of mosquitoes, but 1.2 million is presented as a precise figure when the actual number would vary based on the mosquito feeding amount.