CREATE TABLE table_20935 ("Year  ( January ) " FLOAT, "Population  ( 000 ) " FLOAT, "Rural ,  %" FLOAT, "Urban ,  %" FLOAT, "Source" TEXT)

For how many years was the urban population 57%?
SELECT COUNT("Source") FROM table_20935 WHERE "Urban, %" = '57'
SELECT COUNT("Source") FROM "table_20935" WHERE "Urban, %" = '57'