I don't think flow cytometry is the right way to count cells, because the cells will be discarded once they get in the flow cytometer.
We use Hemacytometer for cell counting. Basically collect cells from the flask, take a small volume and count the cells using the chip. Once you know the cell count in a small volume, you can calculate how many cells in the total volume. See the following link for the detailed method:
I think you have to choose a diatom cell surface antigen and then find a commercial fluorescent dye-conjugated antibody to do the binding with. Then, you will be able to count your cells. You can follow the procedure adopted for lymphocytes and apply it to your kind of cells.