According to senior US military official who is uncertain on the numbers says, that at least 50,000 IS fighters have been killed since offensive begin by coalition forces since 2014. Overall , it is the success of air-campaigns against IS forces, some 16,000 air- strikes have done heavy damage to IS attackers.
It appears that IS is running out their guns, missiles and ammunition. But still it's not a condition to rejoice and relax.
As per the Coalition forces, "It doesn’t mean that it’s not still an extraordinarily dangerous situation. They are not going to go quietly, but they are going to go. Does not use a casualty count as a measure of effectiveness in the campaign to ultimately defeat (IS) in Iraq and Syria."