Here’s a simple tip to add the week number in your Google Calendar via the “Interesting Calendars” selection.
From within Google Calendar click the drop down arrow to the right of the Other calendars item in the left-side column and select Browse Interesting Calendars.
This will take you to the Interesting Calendars screen where you can select More from the top of the screen.
This then presents a list of available calendars (Holidays, Phases of the Moon etc.) and at the bottom of the list is Week Numbers.
Click the Subscribe button to the right of the screen and then use the Back to calendar link at the top of the screen to return to the calendar.
You will now see the Week Number displayed in the Monday column when in Week or Month view.
You can verify that the weeks are counted correctly by viewing the month of January and checking that it starts with week number 1.