I’d probably recommend looking at this forum post for more info about beta invites right now: WoW Classic Testing Access as of May 22
TL;DR it looks like the amount of beta invites has slowed down or possibly stopped for now, though they’ll likely reevaluate later on down the line. The dev team looks for a variety of factors on beta invites, but you do need to be opted in and have game time to be considered.