Look at the manual for the specific unit that you are thinking of buying.

Greg, is of course right. You can find that detail also in the specs online.

However, 40A@240V is typical for normal double ovens.

I googled and the first one I came across was 50A. Maybe it was abnormal but it would be better to run wire that's too big and just have to down-size the breaker as opposed to running wire that's too small and having to rip it out and start over.

(If the actual double oven is unknown at this point)

My single electric oven requires 50A@240 volts, not that it uses anywhere near that much power.

But this is why you have to read the instructions for the unit you're installing.