Truth be told, me personally Khan and AEW having a pro wrestling event at Wembley is impressive in itself.
A lot of factors can help them, ticket prices(tickets go on sale in May) depending on how much, Familiarity (I know Jacksonville Jaguars don't play in Wembley when the NFL goes over to Europe, and Khan has a football [soccer] team) so fans of the Jags and His Soccer team more or less are familiar with his wrestling company, and The Smark UK Audience (All Those YouTube Wrestling European Channels are in heaven right now) who love wrestling.
Seeing as how AEW highest attendance was 20, 180.
I'd say Khan should go for broke, and get as close to hitting 90,000 as possible. But again, Wembley Stadium is impressive in itself