Provide the number of users who took part in the "mental health survey for 2016".
mental health survey for 2016 refers to SurveyID = 2016
SELECT COUNT(DISTINCT T1.UserID) FROM Answer AS T1 INNER JOIN Survey AS T2 ON T1.SurveyID = T2.SurveyID WHERE T2.Description = 'mental health survey for 2016'