I don't have a super specific definition, but from my view there are ~30 #1 starters and maybe a dozen aces. I see an "ace" as a guy who I'd be comfortable with in Game 1 of a playoff series. I guess that's the simplest way to put it.
I would tend to agree with you on the "#1 starter" definition, but I'm less enthusiastic about the "ace" definition. That one would end up being pretty elastic, I think and stretch out to more than the dozen you're thinking about in your head. Example: I think a lot of Twins fans would be very comfortable with Berrios starting Game 1 of a playoff series, but aren't ready to call him an "ace".
Berrios is a guy that I think many if not most teams would look at as a #1 starter, even if their scouting department would call him a #2. (one thing I learned from The Ringer's series on scouting was that most scouts don't project to the extremes and won't go out on a limb to put someone into more elite territory) Is he one in Boston or Cleveland or Houston? Not on their staffs, but what about New York? or Oakland or the Los Angeles Angels of Anaheim in Southern California Disneyland?
Regardless, the Twins are getting into much better position with their starters and getting closer to having a staff that can perform not only over the long haul of a season but in a short series. Berrios is terrific and will leave people drooling for years with that curve.