Dryfiring without something in the chamber is not recommended by the manual.
That said snap caps take the place of the cartridge. So in theory this should be ok since the firing pin is making contact with a replica cartridge made to factory specs for ammo.
One should be able to continuously pull the trigger with the snap cap in place in the chamber over and over. If not, the slide will have to be racked for each additional firing and the cap replaced in the chamber or magazine and then have the slide racked back and let go.
I have with certain pistols put two or three snap caps stacked in the magazine and then just racked the slide for each additional shot until the caps needed to be reinserted into the chamber. Just reloaded the mag with several and went back at it.