6 is correct. chlorination at C-1 makes C-2 a chiral center, so that will give 2. Chlorination at C-2 will not create any chiral centers (so 1 here). Chlorination at C-3 will make C-3 a chiral center (2 here) and chlorination at C-4 will not make any chiral centers (1 more here). So a total of 6. (But all 6 are not stereoisomers of each other I guess since they have different arrangements of bonds. I think the question might have been "how many isomers result, counting stereoisomers separately?" no?