The Air security missions achievement requests only level 1 missions and shortly after you complete the tutorial you open up your level 2 agents. R&D, Distribution and all other mission types are ok. It should be the total number of security missions done regardless if the level.
Several factors go by the number completed
Like storyline missions. To get those you have to do 16 missions of the same faction and level.
Example, if you do 8 caldari navy L2 missions, and 8 amarr navy L2 missions, you will NOT get a storyline mission.