Sample size query

I have been asked to devise an experiment related to cells in a solution.
A sample of cells will be provided and I must test whether or not there is a difference between the top. middle and bottom of the solution (the theory being that the cells may settle out of solution)
The RSD of the method used for counting the number of cells is 15% and the average number of cells is expected to be approximately 1E+6 cells/mL.
Is there an easy(ish!) way I can determine how many samples I would need to take from each sampling point to show that there is/isn't a difference between them?
Thanks for any help!
In order to calculate the sample size, you need to come up with some hypothesized estimates. May be do a literature search to reach a conclusion on what percentage of cells you might see at the top, middle and bottom (I assume your outcome is proportion of cells in each part). Once you come up with historical or your own educated guess estimates, you just need to plug it into the formula to calculate the sample size

Jenny Kotlerman