There is no theoretical or hard limit on the number of routers in a single OSPF area at all. In fact, I have seen some large networks with hundreds of routers in a single area--200 is quite common, and I've seen many more. There's nothing in OSPF itself that would limit this number.
The primary limitation is going to be based on the amount of memory and processing power of the smallest router in the area. If you have a single 800 in the area, for instance, your entire area is going to be bound by the memory in that 800; if the 800 can only hold 2000 routes in memory, then you can only put enough routers in the area to generate 200 routes.
Note that things like incremental SPF:
http://www.cisco.com/en/US/products/sw/iosswrel/ps1829/products_feature_guide09186a008012db76.html
will decrease the processing requirements, by reducing the amount of SPF that must be run, in the general case, although the extreme case would be the same as not running iSPF.
At any rate, all of these old "hard limits" came out of old design docs, and are based on routers like the MGS and AGS+. If you are deploying something newer than AGS+' in your network (and I hope you are), you should be fine with many more than 40 or 50 routers in a single area. Again, I've seen hundreds, with thousands of routes in a single area.
Russ.W
