Dear student
There are 6 isomers which are possible for dibromonaphthalene, considering, each ring to contain one halogen atom.
These are :
1,5dibromonaphthalene
1,6 dibromonaphthalene
1,7 dibromonaphthalene
1,8 dibromonaphthalene
2,6 dibromonaphthalene
2,7 dibromonaphthalene
Regards