Complex sentence: Eventually, the album sold about 15,000 copies which Jugoton considered a failure, unlike Azra whose debut album which is considered to be the most successful debut in the history of Yugoslav rock.
Split into simpler sentences: Eventually, the album sold about 15,000 copies which Jugoton considered a failure. In contrast, around the same time Jugoton released Azra's debut album, which did great commercially and is considered to be the most successful debut in the history of Yugoslav rock.