International Centre for the Study of Radicalisation

A 2018 study by the International Centre for the Study of Radicalisation, at Kings College London, cited by the BBC, put the total number of foreign ISIS fighters at just under 41,500 from 81 countries, almost 6,000 of whom it says came from Western Europe.

Thousands of ISIS recruits from the West:

"Jihadi John", the masked Islamic State of Iraq and the Levant (ISIL) militant linked to the beheading of Western hostages, was named on Feb. 26 as Kuwaiti-born London computer programmer Mohammed Emwazi by experts and the media.
      

Jihadist attacks around the world in November killed a total of 5,042 people, showing Islamist extremism is "stronger than ever" despite Al-Qaeda's declining role, a new study published on Dec. 11 said.
      

Albanian citizen Idajet Balliu, 24, was reported killed in fighting near the Syrian city of Aleppo on Friday - the latest of ethnic Albanians who have died fighting as part of various Islamic militant organisations since the Syrian conflict started.

Two thirds of the rebel fighters in Syria are citizens of European countries, says a report of the London-based International Centre for the Study of Radicalisation. 

According to the report, entitled "Greenbirds: Measuring importance and influence in Syrian Foreign Fighter Networks'", around 10% of the foreign fighters are from Eastern Europe, Bulgaria included.