100% confluent 10-cm dish contains 8.8 million HeLa cells according to some published data. As COS7 cells are slightly bigger than HeLa, I was wondering the figure for COS7 will be less than HeLa.
While calculating confluency you also should consider the cell size. A basic formula will be 'cell count at 100% confluency = Surface area of the flask/size of the cell'. considering COS7 cells are around 2.5 times bigger than HeLa my guess will be a 10 cm dish will have 3.5 million COS7 cells at 100 % confluency.