I've seen folks on 80 planet maps holding 25+ planets in a fallen spire campaign, but it was a lot later on in, there.
I'd really suggest 40 planet maps as the general lower end of "normal" gameplay for the game in general (things just get cramped below that), but 80 is really what most of the game was designed with in mind. For the Fallen Spire, that was Keith's thing and so I can't comment as well on exactly what he thinks would be ideal there. But from what I've seen, I'd guess that having 60 or 80 planets would only be a good thing with that sort of campaign.