Hi,
There shouldn't be a problem with running more than or less than 25 routers in the backbone area. Some of the things to keep in mind however are link stability in the backbone area and summarization. A really good practice is to have your routes summarized before they enter the backbone area, therefore changes in the topology in external areas will not impact your entire domain. Another thing to remember is that E1/E2 type routes cannot be summarized. Your backbone is merely a pass through for other areas. The more stable your links are, the less spf will have to be calculated. Some other factors to keep in mind are CPU and memory requirements. A good book to have is OSPF Network Design Solutions by Thomas. Chapter 5 outlines the suggested routers per area and CPU/memory requirement guidelines to follow. From my experience, these can vary greatly depending on how your network is layed out.
