I don't think there was ever a time 10 was okay. All I've ever heard was 6 and now 1. I suppose it's possible that there was a time period that it just wasn't addressed so you could find anything.

But really, circuit breakers didn't come about until the 50's/60's and by the 60's I'm pretty sure it was the 6 throws of the hand rule. I seem to remember even the old Pushmatic panels from the 50's/60's had a 'sub-main' and fell under the 6 switches.