When  87 mpg-e (39kw-hrs/100mi) is the  epa rated combined fuel economy how many operating modes are there?
CREATE TABLE "table1_24620684_2" (
  "vehicle" text,
  "operating_mode" text,
  "epa_rated_all_electric_range" text,
  "epa_rated_combined_fuel_economy" text,
  "clean_electric_grid_california_san_francisco" text,
  "u_s_national_average_electric_mix" text,
  "dirty_electric_grid_rocky_mountains_denver" text
);
SELECT COUNT("operating_mode") FROM "table1_24620684_2" WHERE "epa_rated_combined_fuel_economy"='87 mpg-e (39kW-hrs/100mi)';