I have a web site with lot of pages and different topics, that I must modifies often.
There are two alternatives:
A unique Sparkle project with all the pages and topics, with a lot of internal links
Various Sparkle separate projects (different folders) and a lot of links (external) between them.
I found more useful the second alternative, because is easier to work in smaller project.
My website was split up into 28 different project files at one point. This was not by choice. Rather, it was because the version of Sparkle that I started using (can’t recall the version number) did not allow for folders and subfolders. When this became possible, I hunkered down and spent a full week merging the 28 projects into one (about 300 pages at the time).
With separate project files, a few things become more difficult or even impossible. For example, editing header or footer would need to be done multiple times. Same with editing styles. And implementing search would be impossible. I recently had to update the publishing credentials due to a change on the ISP side – thank goodness I didn’t have to do it 28 times.