here is a formula to show a weeknum in sharepoint (no built-in function).
=IF(ROUNDUP((ROUNDDOWN([Date],0)-(DATE(YEAR(ROUNDDOWN([Date],0)),1,1))+WEEKDAY((DATE(YEAR(ROUNDDOWN([Date],0)),1,1))))/7,0)>52,1,ROUNDUP((ROUNDDOWN([Date],0)-(DATE(YEAR(ROUNDDOWN([Date],0)),1,1))+WEEKDAY((DATE(YEAR(ROUNDDOWN([Date],0)),1,1))))/7,0))
the formula returns always up to 52 weeks, and the week that has 1st January is the first week in a year. the week starts on a weekday depended on your SP server settings.
if you need to use the 53 weeks standart, use this part only
=ROUNDUP((ROUNDDOWN([Date],0)-(DATE(YEAR(ROUNDDOWN([Date],0)),1,1))+WEEKDAY((DATE(YEAR(ROUNDDOWN([Date],0)),1,1))))/7,0)