How many lone pairs of electrons are assigned to the nitrogen atom in ncl3?

First of all add the number of valence electrons present on molecule,

                       Nitrogen  =  5

                    Chlorine-1  =  7

                    Chlorine-2  =  7

                    Chlorine-3  =  7
__________________________
Total                               =  26 valence electrons

Start making structure with placing Nitrogen atom in center surrounded by three Chlorine atoms.
And, then make single bonds of Nitrogen with all three Chlorine atoms and subtract 2 electrons per single bond from total valence electrons,
Therefore, for three single bonds 6 electrons are subtracted from total valence electrons,
                                                     26
                                                   -   6
                                               _______
                                                     20 
Now, divide these 20 electrons on all atoms starting from most electronegative atom (Chlorine). So, 18 electrons are placed on Chlorine atoms, the two electrons left are placed on Nitrogen.

Result:
           Hence Nitrogen contain one lone pair of electron as shown below,