What is the number for t c (k) when the notation is tl-2212?
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 COUNT("t_c_k") FROM "critical_temperature_t_c_crystal_structu" WHERE "notation"='Tl-2212';