You can tweak your formula to make sure it works over the years. Here's an improved version:

=WEEKNUM([@dates] - WEEKDAY([@dates], 2) + 1)-WEEKNUM(DATE(YEAR([@dates]), MONTH([@dates]), 1) - WEEKDAY(DATE(YEAR([@dates]), MONTH([@dates]), 1), 2) + 1) + 1

This function calculates the nearest Monday for each date. It also adjusts each date to the closest Monday or moves backward to the previous Monday if needed. It also finds the first day of the month and calculates the Monday that's closest to or following that day. It also calculates the week difference relative to the first Monday of the month.