Any number is likely to be a guess at this point but whatever it is, it's likely to be much too high. The Guardian:
Syria is facing increasingly fierce international condemnation for its handling of months of demonstrations against President Bashar al-Assad's rule, partly inspired by the Arab Spring uprisings that have swept across North Africa and the Middle East. Syria says it is fighting foreign-backed "terrorists" and on Thursday announced that more than 2,000 of its security forces had been killed in the unrest. The British-based Avaaz rights group said it had collected evidence of more than 6,237 deaths of civilians and security forces, 617 of them under torture. At least 400 of the dead were children, it added.