How many respondents who participated in the survey in 2019 have ever sought treatment for a mental health disorder from a mental health professional?
respondents' and 'users' are synonyms, have ever sought treatment for a mental health disorder from a mental health professional refers to AnswerText(SurveyID = 2019& QuestionID = 7) = 1
SELECT COUNT(T1.UserID) FROM Answer AS T1 INNER JOIN Question AS T2 ON T1.QuestionID = T2.questionid WHERE T1.QuestionID = 7 AND T1.SurveyID = 2019 AND T1.AnswerText = 1