A dielectric cell with nano material as medium is connected in series. The typical impedance of cell is in terms of Mega Ohms. Now how to know the sample capacitance?

 

A dielectric cell with nano material as medium is connected in series.

In series with what, a resistor?

The typical impedance of cell is in terms of Mega Ohms.

Do you really mean impedance (at a certain frequency), or do you mean the leakage resistance of the dielectric?

Now how to know the sample capacitance?

You could possibly calculate it, or measure it. Some more details like the geometry of said cell and characteristic of the materials used would be of tremendous help.

 

If you expect a good answere you have to do make a detailed question. Otherwise it is a guess.
I assume you have a cell and you want to connect it to some kind of ADC and want to connect a capacitor parallel to the cell.
Select a capacitor with very good isolating material like a foil capacitor for high voltage. If the cell has a DC voltage than the capacitance is not the problem but use a resistor between cell and capacitor to prevent a high current when it is connected first. Or better load the capacitor external to the voltage of the cell to avoid a current out of the cell. The ADC must also have a very high input impedance or you need an opamp with < pA input current as a voltage follower.