Not all scopes share the memory among channels. In other words, some scopes give you the same maximum available record length regardless of how many channels you have turned on.
Also remember that the displays on digital scopes rarely have anything more than 1000-2000 pixels across the screen horizontally. So, even if you capture a waveform using 1Mpoints - depending upon how to view that waveform (zooming in/out, or making measurements, or exporting the waveform for external analysis), the number of points in the waveform may be different that the memory specified for the acquisition.
Some scopes give you direct control over how much memory is being used, and will report this value to you somewhere in the user interface. Some scopes don't tell you easily.
Also note that some scopes use a decimated set of display point for the on-screen measurements, and some use the acquired samples points in memory - this can fool you sometimes too.
Bottom line, you have to take care in how you use the scope to determine the amount of memory used, and how to control it.