Hi, if I understand it correctly, each new registration initiates a new row with information for column A, B, C but what you’re trying to count here is only for column A.
For this problem, I suggest using QUERY. The example can be seen here.
A screenshot:
For column A, I randomly generated the event IDs.
For column C & D, I used QUERY, taking all the unique event IDs (without taking blank cells into account) and count the number of registrations.
Please reply if you need further assistance or I understood the problem incorrectly.