Add file number to beginning of each file in directory

Question

If I have a directory, how do I add the number of the file to beginning of the filename? For example, the directory would look something like this:

file.txt
another_file.txt

And the expected output would be:

1_file.txt
2_another_file.txt

Is there a way to do this? I've seen this post, but I amn't sure how to change the prefix for each file (in this example, have it increase by one each time).