How do I find the number of cores needed for CPU if I know the expected transactions per second?

We are in a scenario where we have a rough idea regarding the expected number of transactions per second in the database server. We don't have the actual (custom built) software in hand. When planning for hardware, how do I determine the number of cores for the CPU for the server in which the software will be housed? Is there any guide or best practices from which I can determine the number of cores for the CPU for the latest processor in use right now?