Clearly, it's virtually hard to determine or even estimate this kind of statistics.
And while it is not known how many of them exist in the world, the house fly (Musca domestica Linnaeus), which originated on the steppes of central Asia, now occurs on all inhabited continents, in all climates from tropical to temperate, and in a variety of environments ranging from rural to urban.
It is commonly associated with animal feces, but has adapted well to feeding on garbage, so it is abundant almost anywhere people live. This species is always found in association with humans or the activities of humans.
The house fly has a complete metamorphosis with distinct egg, larval or maggot, pupal and adult stages.
Warm summer conditions are generally optimum for the development of the house fly, and it can complete its life cycle in as little as seven to ten days. However, under suboptimal conditions the life cycle may require up to two months.
Adults usually live 15 to 25 days, but may live up to two months. Without food, they survive only about two to three days. Longevity is enhanced by availability of suitable food, especially sugar.
To learn more about house flies, refer to helpful links below:
entnemdept.ufl.edu/creatures
www.who.int