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The more subs you take the better within reason, but longer subs are better than shorter subs. With short subs there may not be much level difference between the signal and the noise, with longer subs the signal increases linearly (to a point) but the noise only increases with the sq rt of the signal (this is not an accurate statement and I'm sure someone will jump on it but you get the point i hope. i.e signal = 100 noise = 10, signal = 1000 noise = 33.3 ) The same rule applies with combining multiple images; 4 images = 4xsignal but 2xnoise, 10 images = 10 x signal but 3.3 times noise. Signal starts to swamp the noise.
Except in the dark areas of your image where there is not much difference between signal strength and noise.
Are you shooting in RAW mode and are you using in camera noise reduction? If not are you taking dark frames and subtracting them from the light frames?
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