There is not a limit to the number of subsites that you can create (not at least by SharePoint’s limitations). You are limited by the restrictions of your hardware, or whatever Site Collection quotas your administrator has placed on your site.
You are probably not seeing all of your subsites because your site layout has a width limitation. Another thing to consider is that although you want your users to be able to see your navigation hierarchy, they will have to scroll to the right to do so in your current setup. You may have an easier time of having your users see all of your sites by using drop-downs and flyouts on the top navigation instead of having it be all on one level. (Flyout menus can be configured by editing the master page in SharePoint Designer.) Even if you want your sites to all be subsites of the top level, you can modify the navigation of the site to include nested items.