How many times is the formula tl 2 ba 2 cuo 6?
CREATE TABLE "critical_temperature_t_c_crystal_structu" (
  "formula" text,
  "notation" text,
  "t_c_k" real,
  "no_of_cu_o_planes_in_unit_cell" real,
  "crystal_structure" text
);
SELECT "no_of_cu_o_planes_in_unit_cell" FROM "critical_temperature_t_c_crystal_structu" WHERE "formula"='Tl 2 Ba 2 CuO 6';