### Database Schema:
CREATE TABLE Question (
	questiontext	TEXT	NULL,
	questionid	INTEGER	NULL	PRIMARY KEY
);

CREATE TABLE Answer (
	AnswerText	TEXT	NULL,
	SurveyID	INTEGER	NULL,
	UserID	INTEGER	NULL	PRIMARY KEY,
	QuestionID	INTEGER	NULL	PRIMARY KEY,
	foreign key (QuestionID) references Question (questionid),
	foreign key (SurveyID) references Survey (SurveyID)
);

CREATE TABLE Survey (
	SurveyID	INTEGER	NULL	PRIMARY KEY,
	Description	TEXT	NULL
);

### User's Query:
Provide the number of users who took part in the "mental health survey for 2016".

### Extra Knowledge:
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'