on average an adult has about 4.5-5.5 liters of blood circulating in their body. a female mosquito, when completely full, can hold up to 0.001-0.01 milliliters of blood in their abdomen depending on the species. if we take the average of both (5 liters & 0.0055 milliliters), it would take around 909,090 mosquitos to equal the amount of blood in a single human. although there isnt an exact number of the entirety of the mosquito population, we can use fermi estimation. there is about 57 million square miles of total land area on earth, while say 50 million square miles are habitable for mosquitos. with a rough of estimate of 1 mosquito per 50 square feet (overestimate due to area and time of year). after multiplying the numbers and fixing the units, there is a rough estimated 70 quadrillion mosquitos. theoretically, if a vampire lived in a mosquito dense area, such as brazil, indonesia, malaysia, thailand, etc, and could sustainably hunt around a million mosquitos to fill themselves every time they needed to feed, there would be enough mosquitos to survive on due to their large population and fast reproduction.