CREATE TABLE table_24521 ("Period" FLOAT, "Civilians" TEXT, "Security forces" TEXT, "Insurgents" TEXT, "Total per period" FLOAT)

How many insurgents are there when the total per period is 156?
SELECT "Insurgents" FROM table_24521 WHERE "Total per period" = '156'
SELECT "Insurgents" FROM "table_24521" WHERE "Total per period" = '156'