Libya will probably be kept. It was often referred to as Italy's fourth coast plus the Italians did have the concept when Balbo was in charge of the colony. Without WW2 it is quite possible they can get enough Italians into the area to make it majority Italian especially once oil is discovered. Continued control over Libya also helps Italy secure the central Med, so strategically it is important to Italian interests.
*Ah, round xx*
The Italians at most could get a temporary majority, and then it disappears by the 1970s as Libyan birthrates outpaces that out Italians (see OTL Kazakhstan). The current population of Libya is 7.2 million.
The idea of anything more than a couple million (at the very most) Italians squatting in Libya is very unlikely because:
1) Once an oil industry is set up, you didn't need hundreds of thousands, or even the high five digits, to do operations and maintenance (the Libya National Oil Company has only 65,000 right now, and a lot of those are make work jobs that likely aren't going to be available in even a dysfunctional developed economy)
2) You'd need a leader on the level of Stalin to dump millions of Italians into what's essentially a wasteland. Remember, Italians can make a better living in the industries of the Po Valley (and taking away millions of Italians to do make work in Libya is not going to be good for the Italian economy). And the settlers, and their children, will probably be drawn to go back to Italy, which will have much more in the way of opportunities.
3) There is no potential economic activity that will justify dragging over millions of Italians. Agriculture is not going to employ them, and it'd be ruinously expensive (goodbye, petroleum windfall) to subsidize hardship pay some Italian to clean windows or sort the mail when the Libyans can do it for much cheaper.
4) Building on the previous point, most of Libya is not exactly hospitable to human life. It's a backwater compared to even Sicily in the mid 20th century, and Rome will have to spend a lot to get even hundreds of thousands to come, much less stay.
5) Libyan TFR is guaranteed to be much higher than Italian TFR.
Now, the Libyans might be willing to stay part of Italy after inevitable de fascistization, but the question is, do the voters of a newly democratic Italy want to keep on subsidizing Libya (even with oil money, LIbya will still need huge infrastructure investment for decades to come). FWIW, in OTL, the French like Algerian oil, but they didn't like the idea of all Algerians as equal French citizens.