What is the formula to determine inventory turns per year?

annual sales / inventory

That should be average inventory.  If you are making and selling Xmas
decoration, at yearend, you hope to have no inventory left, but
probably spent all year building up your stock for November/December
sales, and ?turned? your inventory only once.  Even an average of
monthly inventory would give an incorrect turnover in this example
since it might show that you turned over your inventory twice.

Also it should be Average cost of Sales and not Average Sales since
Inventory is valued at cost unless both are in units and not in
dollars