Two clarifications:

1. The suggestion that two electricians gave me is to use a 20-amp or
30-amp double-pole breaker, so there's no need to discuss one vs. two
breakers here.

2. This is a wall oven only on a dedicated circuit. There is a separate
circuit for a cooktop elsewhere in the kitchen.

A rant:

The manufacturer, Frigidaire/Electrolux, is almost useless. Like a lot
of big companies, it has a customer-service organization apparently
designed to block consumers from getting in touch with any
knowledgeable person. The manual and the customer service people just
say conform to your local codes, ask your own electrician, etc., when
presented with a problem such as how to connect the oven's small
16-gauge copper wire to 6-gauge aluminum in the wall, for which nobody
makes a connector.

A conclusion:

As for the circuit rating, I quote from the multiple-model manual:

"The fuse size must not exceed the circuit rating of the appliance
specified on the nameplate."

There is no "circuit rating" on the nameplate. There is only the
wattage, which is 3.4 kw at 240 volts and a similar number for 208
volts, which I guess is for Canada.

Quoting further from the manual:

"The single wall oven can consume up to 4000W at 240 Vac; use a circuit
breaker of 30 Amp with wire gauge #8 AWG."

This figure of 4000 watts disagrees with the 3.4 kw on the nameplate.
So that leaves us with either 14.2 or 16.7 amps.

Thanks for the information about the 80 percent rule. I had not heard
of that. Does that mean that, even on a circuit for a single appliance,
the breaker size should be 1/.8 (1.25) times the maximum expected load
on the circuit?

If I am interpreting that correctly, then a 20-amp double-pole breaker
would handle a load of 14.2 amps multiplied by 1.25, but a 16.7-amp
load multiplied by 1.25 would call for a 20.9-amp breaker, so I'd go up
to 30 amps. In light of the ambiguity of the manufacturer's
information, 30 amps seems best.

Does that make sense? Any other thoughts?