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Please give more details of your specific filter application. In general, the ultimate slope of a low pass filter is 6n dB/octave (20n dB/decade), where n is the number of poles. The more filters you cascade, the sharper the filter. However, keep in mind that cascading 2 identical filters will usually not be the optimal solution. For example, a 2 pole Butterworth filter will out perform 2 cascaded lag filters, if stopband attenuation for a given cutoff frequency is your figure of merit.