The type of hardware you require to run Bitbucket Server depends on a number of factors:

• The number and frequency of clone operations. Cloning a repository is one of the most demanding operations. One major source of clone operations is continuous integration. When your CI builds involve multiple parallel stages, Bitbucket Server will be asked to perform multiple clones concurrently, putting significant load on your system.
• The size of your repositories. On large repositories, many operations require more memory and more CPUs.
• The number of users.

See our guide Scaling Bitbucket Server for sizing guidelines based on the 3 factors above.