Sibelius will automatically flow the music so that it uses what it considers to be the optimum number of bars per system- it's usually pretty good at this, but a little adjustment is sometimes needed to get the result you want. How many bars per system appear on an empty page is irrelevant to the final result. We usually recommend that users input everything first, then think about the layout and appearance afterwards.
Remember that First is very much a cut down version of the full product and there are a lot of things 'missing'.
It may be possible to create a Manuscript Paper that has an empty page showing 8 bars per system, by using line breaks and then saving as Manuscript Paper, or perhaps by exporting the result as a House File (assuming that First supports either of these options) but I can't see any point in doing this, bearing in mind my first paragraph - unless you want blank manuscript paper with 8 bars per system.