I have seen 4 to 5 Administrator accounts in a domain. What is the upper limit on the number of administrator user accounts can be created in a domain?
While you can create 2,147,483,393 unique entries in a domain, you probably won't create even a fraction of this in most environments.
Regardless 2,147,483,393 is the only hard limitation you have in AD.
When it comes to members in a group, there are soft limits which are simply recommendations based on stress tests done by Microsoft and practical use cases in companies around the world.
The highest amount of users in a single group that Microsoft has reported is 500 million, so if you exceed this number you might want to call them and let them know you've officially won the rights to call yourself the active directory king.
Regardless, you're unlikely to reach any real active directory limitation in a company where you currently have 5 domain administrators.
As per the Microsoft Documentation, there are maximum recommended numbers of users per group (5000 is their guideline). Based upon how you intend to scale out AD, and design its structure in the first place, you may want to consult the above documentation to check the limits beforehand.