2-page spread in softcover photo book

Yes you can. I just tried with the Travel Shots theme. Just make sure, you have one of the inner book pages selected. Then click the layout options and scroll down all the way to the spread layouts. If you have the title page selected, the spread layout will not be available. The layout options can be scrolled, even if you cannot see it, because you did not enable the scrollbars.

Yes it does depend on the theme - themes control the page layouts offered

As to number of pages I find that surprising and it makes no sense but if it is a restriction in some or all themes then it is - but I see no reason for a restriction like that

LN

but, more important, it seems that you need to have more than 20 pages on the book to have that feature in the layout options, appear

I have never seen that limitatation. That is interesting. Which theme are you using?

I tested with Travel Shots, and the book allowed the spread pages with the minimum number of 20 pages.