I think the numbers shown on the map may be very conservative estimates, and I doubt they cover the mercenaries hired by Saudi Arabia, Qatar, etc. to fight the Syrian army. But they give a rough idea of the scope of the situation. From The Washington Post; the map is from October 2014; the text in the lower right lists small numbers of fighters that are from 11 countries not shown on the map. The small print is easier to read in the larger depiction of the map at the Post page:
An estimated 15,000 militants from at least 80 nations are believed to have entered Syria to help overthrow the regime of President Bashar al-Assad according the CIA and studies by ISCR and The Soufan Group. Many of these fighters are believed to have joined units that are now part of the Islamic State. Western officials are concerned about what these individuals may do upon returning to their native countries. Related story.