I'm working on 2d game for android in unity and I'm curious Does a large number of objects in the scene affect the performance of the game?(if they are not in the camera frame. for example: different levels)
Yes the number of objects will affect your frame rate.
Assuming that you don't have any scripts running on them their impact on your system will be smaller while they are out of the cameras frame.
You can measure the your current performance with the profiler and use some tips to reduce the load.