For JIRA, see https://confluence.atlassian.com/enterprise/jira-sizing-guide-461504623.html (there's no need to worry about your agents on top of your normal users - think of them as users who do some different stuff but don't add more or less load than a normal user)
For Confluence, https://confluence.atlassian.com/doc/server-hardware-requirements-guide-30736403.html is a good start
It might be. Sorry that this is a vague answer.
You can install the whole Atlassian stack on one machine, and see it work perfectly. The main problem with that is not "doing too much", but "single point of failure" and "We need a machine with enough resources to run all the things"
It is fine to run it all in one place. As long as that place can handle the load.
I have multiple projects that use variations of the same base workflow. The variations depend on the requirements of the project or issue type. The variations mostly come in the form of new statuses ...
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