A dull knife is more dangerous than a sharp one here as it's more likely to slip. If you only sharpen one knife in your kitchen, make it the one you use for this.
If the hinge tears, nothing's ruined. You've just made two cutlets instead of one butterflied piece. Either way, it's ready to cook.
Repeat with as many breasts as you need. The technique doesn't change whether you're doing one or a dozen.