The latest Soufan Group report, which going by Twitter is getting plaudits and their main point is:
In the 18 months since The Soufan Group released its initial report on foreign fighters in Syria, the numbers of people going to fight in Syria have more than doubled, from an estimated 12,000 to between 27,000 and 31,000
Link:http://soufangroup.com/tsg-intelbrie...ers-an-update/

Twitter indicates the full report has many graphs etc.