First of all, you don't need to store "copies" in your table. Why? Because you can always get it in
SELECT
statement.
SELECT MasterAccessionNo, COUNT (MasterAccessionNo) AS Copies
FROM YourTable
But if you still want to store "Copies" in your table, use UPDATE command as _TR_ wrote.
UPDATE YourTable YT
SET
YT.Copies = YT1.Copies
FROM YT INNER JOIN (
SELECT MasterAccessionNo, COUNT(MasterAccessionNo) AS Copies
FROM YourTable
) AS YT1 ON YT.AccessionNo = YT1.MasterAccessionNo