Jamie Dettmer summarizes a new report that sheds light on the question:
IHS Jane’s Charles Lister, an insurgency expert and author of the analysis, estimates that around 10,000 are jihadists fighting for al-Qaeda affiliates (the Islamic State of Iraq and the smaller Jabhat al-Nusra), while another 30,000 to 35,000 are hardline Islamists, who have less of a global jihad vision but share a focus on establishing an Islamic state to replace Assad. Another 30,000 or so are more moderate Muslim Brotherhood Islamists. He estimates that moderate nationalist fighters number only about 20,000, with the Kurdish separatists being able to field only 5,000 to 10,000. …
On his Twitter feed, Lister concedes that it is a “rough science” to estimate rebel numbers and assess their ideological coloring, but he says he has based his calculations on open sources as well as intelligence assessments, and on interviews with opposition activists and militants. He notes that while the al Qaeda affiliates don’t have the largest numbers, “they they have the most resources and best weapons, and they have very good organization.”
Eli Lake reports on fighting amongst the rebels:
The same day the United States and Russia announced a plan to disarm Bashar al-Assad of his chemical weapons, a fresh round of fighting erupted along the Syria-Iraq border. This time, it was rebel versus rebel—specifically, al Qaeda-linked rebels against the more moderate elements of the opposition. … [T]his weekend’s clashes—which came after a Sept. 12 messagefrom al Qaeda leader Ayman al-Zawahri instructing his followers in Syria not to collaborate with the FSA councils—could mark a more violent stage for the opposition’s fractured ranks.
Earlier analysis here.