Calendar app for iOS by default does not show the week numbers. It is an easy switch to display the week numbers in the calendar view. This is useful if your work rely on week numbers for event planing and schedule.
Go to Settings app > Mail, Contacts, Calendar. Scroll down to the Calendars section and toggle to turn on the “Week Numbers” switch. The week numbers will appear in a lighter grey text on the leftmost side before the start of the week.