How many frequencies have a line of East London and destination of Crystal Palace?
CREATE TABLE "services" (
  "platform" real,
  "frequency_per_hour" real,
  "destination" text,
  "service_pattern" text,
  "operator" text,
  "line" text
);
SELECT COUNT("frequency_per_hour") FROM "services" WHERE "line"='east london' AND "destination"='crystal palace';