How many people is that, do they pack the bus all seats sold in order to be “sold out”? Not asking about small group.
A full tour is usually 40, unless it's a small groups tour. Some tours have a max less than 40 and if so, it will say that in the description.
Sold out doesn't always mean a full bus. There may be other conditions that lead to a designation of sold out. The last two tours we went on were listed as sold out but both had under 30 people. The first was the Canyonlands tour which I think changed from available to sold out as we approached departure so that they could release the rooms they were holding in the National Parks. The second was the England, Scotland and Wales tour that included the Edinburgh Tattoo. Again I think they released the extra tickets they held for the Tattoo and changed it to sold out.
Italy may be a different situation. I recall having about 40 people on our Classic Italy tour. If you are asking about a tour scheduled for next year, it may have the 40 people. Some will back out when the final payment is due but they will probably be replaced with others waiting for something to open up.
Even with a full bus we didn't find it uncomfortable. The back row or two were open. If it will not fit in the overhead or under the seat, you can bring your hand luggage down each morning and put it under the bus and pick it up when you arrive at the hotel. You should have enough room.