It's entirely possible that there's some no-code product being developed out there that supports connecting to Reddit's API, so that you could collect the information you want without writing an actual program.
But you're on the Software Development Codidact, so I'm going to assume that you're here because you're open to the possibility of developing some software. (If you aren't, this question should be closed as off-topic.)
So broadly, if you don't have any preference for a programming language or other toolkit yourself, what I would recommend doing is this:
- First, find a popular library for accessing Reddit (possibly referred to as a binding or a wrapper for Reddit's API). Here's one for Python; here's one for Go; you can find more yourself. Pick one with documentation that you think you can understand; good documentation is probably the single most important feature of a library for beginners. (It's technically possible to achieve your goal by contacting Reddit's API directly, but that won't be a good project for a beginner.)
- Follow that library's instructions for installing it, if applicable. For Reddit, I think any library is going to require you to sign up for a Reddit developer key, and you will also need to read about Reddit's API usage restrictions.
- Write a program in the language supported by that library. In any general-purpose programming language, the program itself should be a good beginner's project. Use either the documentation provided by the library you've chosen, or Reddit's API documentation, to figure out how to get the information you want about the subreddits you're interested in. Use the language documentation, or tutorials that you might find for the language, to figure out how to represent or read a list of subreddits and perform an action for each one.
(I would not recommend attempting this in Excel. At the very least, you would need to write VBA code in order to make Excel contact Reddit, and you probably won't find a VBA wrapper for Reddit's API, so you would have to write a lot of extra code yourself. Possibly worth it if you had programming experience, were looking for a challenge, and had some compelling reason to publish this information specifically as an Excel spreadsheet, but it doesn't sound like any of those are true.)
If you have follow-up questions about specific issues you encounter while embarking on the above, search for their answers yourself, and if you come up empty, feel free to ask them here.