By extrapolating from the 25,000,000 inhabitants of Italy at the time of unification, natural birth and death rates, without emigration, there would have been a population of about 65,000,000 by 1970. Instead, because of emigration earlier in the century, there were only 54,000,000.
[60]
From Wikipedia, fertility dropped off sharply in the 70s but momentum kept the ethnic Italian (including Aoste-south Tyrol) growing in the 80s, then it stagnated/slightly dropped, assuming similar birth rates there would be about 68 millions Italians without significant immigration or expansion taken into account, of this Italy had the same immigration as IRL that would amount to 72-73 millions people.
Without specifying a pod, I think it’s not impossible for Italy to have a similar immigration as France, after all most of france’s migrants came from Africa (Maghreb and subsaharian) and Southern Europe, now Italy is a bit further away than France from Iberia, if France developped more slowly maybe many provençaux may go to Italy, France has 12 millions people who were recent (themselves or their parent) migrants, i’m Not sure but i’ve Seen numbers around 52-53 millions ethnic French without counting post ww2 immigration, in this case there would be around 14-15 millions people of migrant background, if you can apply that to this Italy they would have about 82-83 millions people today, which is close to the current german population, this Italy likely would have a history of involvement in Africa, many colonies, and effective control of North Africa, it’s hard to imagine a colonial empire and large as france’s in subsaharian Africa but maybe Italy could get more migrants from East Africa, Egypt and the levant to compensate. This would also require some kind of retornados/pieds noirs from North Africa , from Libya for exemple, except if they can manage to keep it.
Now you can maybe add additional territory surrounding Italy, Slovenia, istria, Malta, lcoastal Croatia, savoy.... and it’s maybe possible to have an Italian population around 85-90 million