They'd need your whole hand, since any finger can be used for a smartphone biometric scanner.

Truthfully, lots of different scannable areas will unlock a phone. I've successfully and reliably configured my toes, the knuckles on my hands, and the tip of my nose. All of them work pretty good.

Unless observed using biometric security in a specific manner, an adversary might have a hard time deducing what kind of print will provide access.

Even if they've determined that the phone contains biometric scans tied to security, how would they know it's yours, and not someone else's, or even a specially printed 3D key ring fob or something?

Then again, criminals don't always think deeply about such details during a crime. They might just chop off both hands, grab the phone, and figure out the rest on the run.