Maximum storage capacity in a reflection-type holographic memory with three-dimensional speckle shift multiplexing is investigated numerically. An explicit expression of storage capacity is derived on the basis of interpage crosstalk noise. We fabricate a simulator to evaluate reflection-type holographic data storage by calculating wave propagation, recording a hologram, and reconstruction by scalar diffraction. We calculate the properties of the resultant diffraction efficiency, that is the noise, at the first null in the speckle-shift multiplexing. Numerical results indicate that the storage capacity is proportional to the numerical aperture to the fourth power and to the volume of the recording medium and is inversely proportional to the wavelength to the third power. Achievable storage capacity is discussed.