Divergence angle of the performance laser beam is about 1.4 degrees. We can figure out how much this matters.
Cardstock is kind of thick, it varies but about 0.28mm per sheet is a good guess for 100lb cardstock. That means that we’re talking about 2.8mm depth change across ten sheets. The beam radius therefore increases or decreases by about 0.0027” in that vertical travel — not much.
We don’t need to guess about this or rely on theory though, we cut ~3mm material all the time and it’s been quantified:
Anyway. Do I think that cutting multiple layers of cardstock is a good idea? Not really, but not because of fire risk: I think you’ll get significant smoke staining between the layers. You can deal with fire risks but smoke is a much harder problem to solve for. You can mask cardstock (I’ve done it and it works well) but it’s a lot of labor. You could probably get away with masking one side of the back won’t be visible but still.
If it were me I’d probably do them one at a time. I hate smoke marks too much to fool with it.