Can you clarify your requirement slightly? I am not sure which interpretation is right. I will describe a couple of interpretations using an example scenario with 2 producer applications rather than 20 for simplicity, and I will use T=? to represent some point in time, where T=1 happens before T=2, etc.
In application 1:
T=1, event A11 occurs on customer 123
T=4, event A12 occurs on customer 123
In application 2:
T=2, event A21 occurs on customer 123
T=3, event A22 occurs on customer 123
Interpretation 1:
Subscriber must receive messages in the order A11, A21, A22, A12
Interpretation 2:
Subscriber must receive A11 before A12,
and it must receive A21 before A22,
but not necessarily with a total order of A11, A21, A22, A12
If the requirement matches interpretation 1, then the desired result cannot be achieved unless the events in application 1 and application 2 are causally connected (in which case you can use vector clocks).