Here's a simple workaround on how to show week numbers in Calendar of your iPhone or iPad devices.
Note: This feature is only available on iPhone, iPad, and iPod touch running modern versions of iOS, if you do not see the Week Numbers toggle within your Settings app for the Calendar, you’d need to update iOS to reveal the settings change.
How to Show Week Numbers in Calendar for iPhone & iPad
1. Open the Settings app and go to “Mail, Contacts, Calendars”
2. Scroll down to the “Calendars” section of the preferences
3. Toggle “Week Numbers” to the ON position
4. Open the Calendar app into Month view to see the change
The week numbers will appear directly before the start of the week in a lighter gray text in Month view and Dates List view, on the leftmost side of the week start date, though it will shift slightly if you changed your weeks start date to Monday rather than Sunday.
Here’s what Week Numbers in iOS Calendar app looks like before and after being enabled: