How many stars you get in your picture depends on the amount of light pollution present, ISO setting and shutter timing. Your 18-55mm, f/3.5 will do as good a job as a 35mm, f/1.8. The only thing a 35mm, f/1.8 will do for you is allow a faster shutter speed or a lower ISO. To get more stars try a higher ISO and/or a longer shutter speed.