Are you using a Directory type mailmerge main document?  In such a document,
you just have one set of mergefields arranged in the configuration that you
want the information to appear.  When you execute the merge to a new
document, that document will contain a set of the information for each
record in the datasouce, with each page being full of information.

If that is the way you have it arranged and you are still only getting one
record of information per page, make sure that the paragraph format in the
mail merge main document is not set to have a page break before it.