How many different PPI does the model with ppcm of 87 have?
CREATE TABLE "retina_display_models" (
  "model" text,
  "ppi_pixels_per_inch" real,
  "ppcm_pixels_per_cm" real,
  "resolution" text,
  "typical_viewing_distance_in_cm" text,
  "pixels_per_degree_ppd" real
);
SELECT COUNT("ppi_pixels_per_inch") FROM "retina_display_models" WHERE "ppcm_pixels_per_cm"=87;