I am afraid your question is too general. It depends on how many routes you have in your network, whether the router has multiple roles (e.g. DR, BDR, ABR), how stable in general your network is. I believe this kind of things are better determined in practice (e.g. you have 10 neighbors, you add 1 more and see what happens).
Maria makes a good point that there are a number of aspects of the OSPF network that impact performance. I would especially emphasize the frequency with which the SPF must calculate. With a very stable network (infrequent calculation of SPF) you can have a larger number of neighbors.
As one observation (to help establish general ranges) I have seen a router (somewhat lower in the product line than a 7600) operating with 200 neighbors and running ok.
We agree with Rick that there are aspects of the OSPF network that affect performance. Just wanted to add that there are even non-OSPF related factors that may affect performance, such as your router also running BGP. So, your general setup is what counts, as well as whether you are extra careful with your network or you sometimes take risks to avoid unnecessary costs.
Perhaps my example with only 10 neighbors was unfortunate, but it was just an example to show that incremental changes in the network can give you smooth feedback on such things. I would not make changes like moving 10 neighbors from one router to another without good reason.
One last thing. Performance issues on routers that have a separate card for control plane traffic are less often.