Here's a cover gallery of my complete runs Yaffas' Kull the Conqueror and Kull the Destroyer series:
Kull the Conqueror #1, c.October 1977
Kull the Conqueror #2, c.December 1978
Kull the Conqueror #3, 1980
Kull the Conqueror #4, 1980
Kull the Destroyer #5, c.March 1981
Kull the Destroyer #6, c.August 1981
Kull the Destroyer #7, c.January 1982
Kull the Conqueror #'s 1 and 2 are regular/magazine-sized issues. The rest of the issues are digest-sized. Kull the Destroyer #'s 6 and 7 are tall digests, the others are short digests.
Each issue reprints three Kull stories from the respective US Marvel series. They are reprinted in sequential order.
Marvel's Kull the Conqueror ran for 10 issues. The numbering continued in Kull the Destroyer. The crossover point in the Yaffa series is in Kull the Conqueror #4. This issue reprints the main contents of Kull the Conqueror #10 and Kull the Destroyer #'s 11 and 12.
Some of the Kull the Destroyer issues reprinted short vintage Marvel stories as page fillers. These stories by the likes of Steve Ditko and Joe Maneely are included in the respective Yaffa issues.
It is possible there is a Kull the Destroyer #8 but I have not seen a copy.