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The end game is exposing long enough to have the faintest regions of the frame above read noise OR not saturating your subject, whichever comes first. The total number of photons that are captured is determined by overall exposure time, not the number of subs nor the length of single subs.
For instance, my QHY 22 collects the same number of photons with 4x150s as my QHY9 did a single 600s; both are sky limited by this point. As read noise drops the sky limited exposure time drops as well. My QHY22 has a read noise of 4.7e- whereas the QHY9 had 8.5e-. At unity gain you're closer to 2.2e- which means that you probably only need to shoot closer to 60s to get above the read noise in broadband.
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