Some people are saying they've heard as many as 7000 traveling without tickets. I've not heard that from anywhere, just saying that some are saying that they have heard that.
Plenty here if you want to take the risk:
https://www.seatsnet.com/en/sports-tickets/football-tickets/brighton-and-hove-albion-vs-afc-ajax/
https://www.livefootballtickets.com/fixtures/ajax-v-brighton-hove-albion-tickets-europa-league.html
I think on top of that, for Europa League games, you need an Ajax Club Card, which you can only buy in pre-season. In addition, their website says the game is sold out.