I am doing a project where I want to build a model that estimates the number of retweets at a given time for a given tweet bearing a certain keyword, so that when a new tweet with the same keyword comes in, I can track its number of retweets and see if there is any anomaly.  For that, on top of collecting a large number of tweets with that certain keyword for modeling, I need to know for each of the tweets what was the number of retweets on day 1, day 2, etc (the unit of time is arbitrary here, can be in days or in minutes) since it was created.  
I've done some research on stackoverflow, but I have not seen a solution for this particular problem.  I understand that twitter API allows you to search for tweets with keywords, but it only gives you the tweets' current number of retweets but not the historical performance.  
I would really appreciate it if anyone can point me to the right direction.  Thank you very much!
There's no API method in any of Twitter's API tiers (standard, premium or enterprise) that would enable you to do this. You'd need to have already been listening for the Tweets, record the Tweet IDs, check them every day for total engagement numbers, and then record the differential every day.
Get Historical Tweet Metadata from Twitter API in Python