Hello, do you want to know the exact quantity or just the monolayer percentage?

If the problem is trypsin, you can remove the cells with a rubber policeman and check that you have removed the entire monolayer, homogeneize the clumps and count cells in a hemocytometer

If you want  to calculate the percentaje of adherent cells, you can use a dye and colour the monolayer or observe cells in vivo,  observations should be made under a microscope with low magnification, dependig on the confluence of the monolayer (10X - 20X)  and then count the areas with and without cells by microscope field, count at least in 12 different zones of the flask, 4 in the middle, 4 in the corners and 4 in the edges.

Dear Celeste Thanks for answer the true is I want to know the exact quantity of my cells in 25 cm flask. For example 2.8 x 10^7 cell (cell density).

 If you are designing your experiment, you can work with replicates and count the replicates withouth using the flask or seed cells in an impedance system.  But If you already have the adherent cells into the flask and you do not want to dettach them, you can use an image analysis system. See attached file.