How many different pairs of candidates were there for the district first elected in 1988?
CREATE TABLE "table1_19753079_12" (
  "district" text,
  "incumbent" text,
  "party" text,
  "first_elected" real,
  "result" text,
  "candidates" text
);
SELECT COUNT("candidates") FROM "table1_19753079_12" WHERE "first_elected"=1988;