Literally, 7 Khoon maaf means 7 murders forgiven. At the same time, it seems that Vishal Bhardwaj's upcoming movie also has Priyanka Chopra that is Susanna killing her seven husbands, so there will be seven murders and that is what the title of the movie seems to be all about.
Though there is one more thing that should be clarified here. 'Saat Khoon Maaf' is an old term which used to mean a bit different from this. Since khoon also means blood, saat khoon used to signify coming seven generations (just like saat pushten), or simply 'all generations' of someone, and saat khoon maaf was used to forgive someone forever, generally by kings and all, as it would mean (all) seven generations forgiven.