It depends on the bore and thickness of the gasket. The increase in volume from one gasket is:

pi*r^2* thickness r=bore diameter/2

If both gaskets are the same add them together and calculate c.r. again

If you know the compression ratio you can figure out the thickness of gasket material you need to add or remove to achieve your desired c.r. There are some c.r. calculators online you can use. Just play with the gasket thickness, get the desired comp ratio and divide the gasket thickness by the thickness of one gasket.