I don't why but in spring and summer time outside the main door of the house, there are usually 15 to 20 mosquitos just lying around. It is so bad to the point we have to avoid using the main door and use the garage for entry. How can I get rid of them?
Bugs will hang out where there's cover -- such as bushes -- and where there is as little breeze as possible. Hungry mosquitos are, as Jasen pointed out, attracted to CO2, so if you have a furnace exhaust or some such near your front door, that could contribute too.
I'm guessing there is some significant difference in the local environment around your garage door -- breeze, vegetation, maybe even amount of sunlight. If you can't change anything around the front door area, well, that's life in reality.