We have a (2) node NS-208 cluster running version 5.4.0r10 and we establish an IPSEC VPN tunnel to 250 5GTs. Our routing protocol has been RIPv2 and we are interested in migrating to OSPF but our consultant has informed us that OSPF has a neighbor limitation of 16ish neighbors in an area. So we would have to break up our environment in to multiple areas. I'm unable to find any documentation to support this either in print or online.
The specifications for the OSPF protocol do not specify any neighbor limitation. Back then OSPF was originally created, routers were not as powerful and breaking up the environment in areas was often necessary to distribute the burden better.
Nowadays you can effortly run OSPF on hundreds of routers in one area. Most likely you are adding unnecessary complexity to the network by dividing it into areas.
To be sure it's advisable to gradually migrate to OSPF and keep an eye on the resource usage of the devices by using a monitoring tool (e.g. Cacti).