In a paper, I am describing subsets of data, but I'm not sure if I should describe these in the past or present tense. Should I use "In the data, there were 56 vowels and 50 consonants." or "In the data, there are 56 vowels and 50 consonants."?
Referring to this writing guide LINK, I would suggest writing in the present perfect tense.
The present perfect is mostly used for referring to previous research in the field or to your own previous findings. Since the present perfect is a present tense, it implies that the result is still true and relevant today.
The present simple could also be used here, but the present perfect focuses more on what has been done than on what is known to be true now (present simple).
If you are in the United States I would refer you to the Modern Language Association (MLA) guidelines as a widely accepted set of academic writing procedures. Though, there are other formats, like the American Psychological Association (APA) guidelines, so check with your specific institution to confirm which is used.