The strength of the magnetic field between the poles of an electromagnet depends on:

The number of turns in the coil. If we increase the number of turns in the coil, the strength of electromagnet increases.

The current flowing in the coil. If the current in the coil is increased, the strength of electromagnet increases.

The material of the core. The strength of the magnetic field increases if the air core is replaced by soft iron. i.e. the strength of the magnetic field depends on the material of the core. 

Changing the direction of current in electromagnet just changes the direction of the magnetic field. Its strength remains unaffected.