, were published

In an attempt to emulate the success that Fleetway had had with girls' magazines, the relaunched Eagle initially contained a large number of photo stories such ashttp://en.wikipedia.org/wiki/Doomlord, http://en.wikipedia.org/wiki/Sgt._Streetwise and http://en.wikipedia.org/wiki/Manix, but this style was soon replaced by the more traditional comic-strip format. Along with IPC's entire comics line, Eagle was sold tohttp://en.wikipedia.org/wiki/Robert_Maxwell in 1987.^{http://en.wikipedia.org/wiki/Eagle_(British_comics)#cite_note-69} Although not as successful as its predecessor, over 500 issues were published.

I think "over 500 issues" is the subject of "were published"; if so, I'd like to know whether preposition phrases can be used as a nominal phrase.

p.s.) I have, until now, thought preposition phrases can be used only as adjective phrases or adverbial phrases.

Thank you in advance for your help.