How many total interview subjects wererthere when the centerfold model was Tamara Witmer?
CREATE TABLE "table1_1566852_6" (
  "date" text,
  "cover_model" text,
  "centerfold_model" text,
  "interview_subject" text,
  "20_questions" text,
  "pictorials" text
);
SELECT COUNT("interview_subject") FROM "table1_1566852_6" WHERE "centerfold_model"='Tamara Witmer';