My teacher told that the number of magnetic fields in a current carrying conductor increases as the current flow increases. So, does the length between two magnetic field lines also increase if there is increased current flow,i.e., if the length between field lines l1 and l2 is 1cm at first, will it increase to a length more than 1 cm in the second case if the same conductor is used? Or does the magnetic field expand only due to increase in no.field lines?
Magnetic field is AROUND the current carrying conductor, not inside it (well technicaly it is, but it radiates outward). The magnetic field around it is traveling away fro mthe conductor at the speed of light reducing in it's strength. This is why strength of magnetic field decreases with distance. Magnetic field lines are fictional and are just used to simplify imagining the strength of magnetic field (more lines in small space mean bigger field strength).
When the current increases the strength of magnetic field also increase, thus if you want to use your teachers analogy, the number of magnetic field lines in a space around the conductor increases.
A stronger field is sometimes represented by more magnetic field lines passing through any given area. So with more current in the conductor the lines will get closer together! But as Ma Drung says, the lines are fictional, insofar as one can draw as many or as few as one likes. In cases like a current-carrying coil, if one decides (arbitrarily) on a number of lines passing through a given area near the centre of the coil, the lines do get further apart as we follow the lines as they curl round the coil – where the field is weaker. [Unfortunately we can't do this for the field lines round a long straight wire, as the distance apart of any two lines stays the same as they curl in circles around the wire. If we want to represent the strength of the wire's field by how close together the lines are, we have to do this 'artificially' by choosing the spacing of the lines ourselves.]