How to know how much usage one person is using and implement a system to allow a fixed amount of usage per subscription tier?

Hi everyone so I have built a program that uses Open AI API, I have a few people interested and realized that some users will be using my company’s (solo dev) API key for more requests than other users and I believe they should not be charged the same. I am thinking something like… ex: "basic plan (100 requests a month) " pro plan (1000 request a month) how do I figure out how much data one person uses? I need this to figure out how many API pulls someone can do a month to figure out my profitability, and what is a fair usage per user. If anyone has any ideas let me know. Thank you.