Hi, I want to hook up a 9.2kw stiebel eltron dhc 10-2 tankless water heater to a Square D QO electrical panel and it requires a 50A circuit breaker and 8-gauge electrical cable (3 conductors?), should I buy a 1-pole or 2-pole plug in circuit breaker? Why? What is the difference between a 1-pole and 2-pole even though both are 50A? Thank you!

You will need a double pole 50 amp breaker and 8-2 with ground cable. Granted that is three wires, two conductors and the ground wire, but the ground wire is not a conductor.
You need a double pole breaker because the heater requires 240 volts. You get 240 volts (in USA) by connecting to the two 120V lines coming into the breaker box.

You must use a double pole breaker, not two single pole breakers because the two 120V lines must be disconnected at the same time.

Note 8-2 w/ground cable is not the same as 8-3 cable.

I have to say, even though I know this will be an unpopular opinion, if you do not know the difference between a 1-pole and a 2-pole breaker, and you are considering #8 cable for a 50A circuit, you will be best served hiring an electrician to do this job.

Amen.

Amen x2

Thanks HKSTROUD for confirming for me which breaker and wire to buy and everyone else for your input. I do intend to hire a licensed electrician to hook everything up so it doesn't go poof! Amen (x2) indeed if I don't *LOL*

Let electrician purchase cable and breaker. Specifications of water heater calls for #8 cable but there are different types of cable. If distance is very much it may be best to use aluminum cable.