CREATE TABLE table_23921 ("Number" TEXT, "Previous BR No." FLOAT, "Taken into deptal stock" FLOAT, "Withdrawn" FLOAT, "Disposal" TEXT)

Name the total number of previous br number for number 21
SELECT COUNT("Previous BR No.") FROM table_23921 WHERE "Number" = '21'
SELECT COUNT("Previous BR No.") FROM "table_23921" WHERE "Number" = '21'