By the end of 2019, according to UN, 386,000, but opposition claims 585,000, & 69,385 are foreign fighters.
Syria has not accepted loss of Golan heights to Israel. After Arab spring, west decided to use it to oust Asad regime. Jordan provided the training camp, Turkey safe border pass for Europe’s Wahabi fenatics, Qatar, Saudi, UAE & other Sunni government financed the war, Israel provided “humanterian” & medical support.
Isis sold oil to Israel through Turkey. US & Israel bombed Asad regime positions many times in support of Isis, until finally Russians decided to support Asad.
Now, at the end of 2019, Isis is crushed. Some fighters are fled & killed & this conspiracy is almost over, despite all US allies will.