Looking for formula to cumulatively count number of unique customers over time

Hi there, I am looking for a formula that can count how many customers have entered the company database over time. I have included an example data set to illustrate what I am looking for. In the example data set, I am looking for a formula that can cumulatively count over time the number of unique customers that appear. For instance, I have included desired results in B8-D8. In Jan 2021, customer 1 purchases an item, so there is 1 customer in the database. In Feb 2021, 2 new customers make a purchase, so the total unique customers comes to 3. In March, only customer 1 purchases again but the total unique customers remains at 3 since customer 1 has already purchased.

if someone could help me find a formula to do this automatically that would be amazing! Thanks!!!

Not a formula, however perhaps this may be useful.
As you have the 2019 version of Excel you can use Get & Transform to put the data in record (row over row) arrangement using the following Power Query Advanced Editor Code:

Please Login or Register  to view this content.

The code produces the records in A10:B19
C10:C19 is added using the following formula: =IF(COUNTIFS(A$11:A11,A11)=1,SUM(C10,1),C10)
A pivot table then displays the cumulative number of customers by month.
Let us know if you have any questions.