Complex sentence: Estimates of the number of SSNP fighters in Syria range from 6,000 to 8,000; according to SSNP's sources, "although Lebanese SSNP fighters are among their ranks, their proportion within the group's total fighting force has decreased steadily as more Syrians sign up".
Split into simpler sentences: Estimates of the number of SSNP fighters in Syria range from 6,000 to 8,000. Their ranks include Lebanese fighters, even though the party claims that "their proportion within the group's total fighting force has decreased steadily, as more Syrians sign up".