If you are thinking about sonar senors then you need atleast 6 of them (four sides + Upside+Downside) this will increase no of devices connected therefore you will need more ports/pins to connect them.
Also you can reduce the no. of sonar sensors by restricting the drone to move in only on direction that means if you want to make the drone a reverse motion then it should turn 360 degree them start moving forward. by doing this no. of sonar sensors will be decreased to 3