Incorrect spelling, explanation: it is wrong to spell after which together, as these are two separate words creating a preposition relative pronoun together. We don’t mash them as one word, this is why afterwhich is incorrect. We should write down this phrase separately: after which.

Correct spelling, explanation: after which is a preposition relative pronoun, where after is a preposition, and which is a relative pronoun. Because these are separate words that create collocation together, we spell them separately – after which is the correct spelling. Writing this phrase together as one word (afterwhich) is a mistake. Other examples of prepositions relative pronoun are by whom, and on what.

Definition of after:
1. preposition (of time), later in time than something else
You will be asked to take an exam after the interview.
2. preposition (of place), next in terms of place
He was standing after me

Definition of which:
pronoun, what one, what ones of (for example a number)
Which film did you like best?
This is the book which I borrowed from the library.

Examples of usage of after which
We are going to have a party, after which you will have many new friends.
Click “book a ticket”, after which check your email to confirm.