Create a new formula field called {Weeks}
with this as the formula:
"1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53"
(Its just a text string that lists all possible week numbers: 1-53)
Set your {Weeks active}
field to be a formula-type field with this formula:
MID(
Weeks,
FIND({Week - Created}, Weeks),
FIND({Week - Finished}, Weeks) + LEN({Week - Finished}&"") - FIND({Week - Created}, Weeks)
)
Note: This won’t work for projects than start at the end of the year (i.e. week 50 of 2021) and finish at the beginning of the next year (i.e. week 1 of 2022).