Hyperloglog, Probability, Hash Function, Cardinality Estimation, Dynamic Programming.
30 days to get 2.6 trillion user ids in a month. Assuming that each user id is of size 32 bytes, we will need 76 TB of data points to analyze and find the number of unique user ids from this data. Ofcourse, one can store all the 76TB on disk and scan the data to find the number of unique user ids