Correct spelling, explanation: this phrase is spelled separately because it is made of two independent words that don’t merge. After which is a preposition relative pronoun such as by who or in where. This is why afterwhich is the wrong way to write this phrase.

Definition of after:
1. preposition (of time), taking place later in time
I have an exam after you.
It’s unhealthy to go to sleep right after eating.
2. preposition (of place), following in terms of place or position, behind
We lined up one after another waiting to enter the club.

Definition of which:
pronoun, what one or what ones of the group
Which dress should I buy? Green one or a blue one?

Examples of usage of after which
First, we are going to have lunch after which we have to take a taxi to the airport.
Tomorrow we have a maths class in the morning, after which we go on a school trip.

Incorrect spelling, explanation: afterwhich shouldn’t be written as one word, that is why this spelling is incorrect. These are two separate words that collocate together: after which. Like other preposition relative pronoun, it is spelled separately.