There have been many discussions regarding this general topic on this forum (most of it probably in the archive). If you find those discussion threads there is a lot of insight as to what others think and have experienced.
Infor has a best practices document which has some basics.
Essentially this has more to do with the number of threads in a process (and by extension the number of processes/threads in a site) rather than any ratio of OB to IB threads.
Having said that, a lot depends on your environment and the workload characteristics in the process(es)/site(s).
Generally (and this is really general) I find 10 threads (combination of ib and ob) per process to be quite acceptable.
Inter-process routing has other considerations as well (such as using cross-process versus ob thread from one process to ib thread in another process via tcp/ip ports). Consideration needs to be given if the licensing is thread count based.
So, in my opinion, there is no real ‘cookbook’ response. One needs to do a thorough evaluation and architectural design as a result of the evaluation with future growth considerations in mind. Taking into account the hardware and O/S potentials as well.
Sorry but I don’t think there is a quick and easy answer to your question.
51 threads in a process (I am assuming here all of the threads you are using in your example are in one process) may work well in your environment for now or may be presenting issues which are not yet manifesting themselves externally.
It is good though that you are taking a look.
email: jim.kosloskey@jim-kosloskey.com 29+ years Cloverleaf, 59 years IT - old fart.