Before we added the new jquery file uploader we had to have separate <asp:FileUpload control for each allowed file so to add more we would have to add more to the page and show/hide them according to how many were enabled. Those still exist and we would be limited to 4 if we disabled the jquery file uploader which can be done by a config setting.
But with the jqueryfile uploader working the number of files is really only limited by the feature setting. You could add more by editing the file /FormWizard/Controls/AllowedUploadsSetting.ascx and adding a few more to the dropdown list there. Then in settings you could choose a higher number.
I will look into a way to make the dropdown show more as long as the jqueryuploader is enabled for a future release of Form Wizard, but by editing the mentioned file I think you could solve it now.
Hope that helps,