"According to new research from Chainalysis, a digital forensics firm that studies the bitcoin blockchain, 3.79 million bitcoins are already gone for good based on a high estimate—and 2.78 million based on a low one."

Now, how accurate was their analysis? I can't answer that.  But this will give you an idea of what the possible number of "lost" coins might truly be.  At least 2 million might be a safe guess.