How many routers and users can be in a single OSPF area?
Well one rule-of-thumb used to be about 50, but as Franco notes, with modern routers you might easily double that, but again, there are many variables involved. The less you have probably makes you "safer". If you want to be super cautious, you might want to stay under 30, yet again, your network might be fine with many, many more. Part of the consideration might be how your topology shakes out, i.e. whether there's natural hiearchy you can map your OSPF areas and/or what advantages routers having knowing the topology of their area. Also keep in mind, OSPF area borders is also where you manage network prefix summarization and/or network prefix distribution.