A post on Apple developer forum is discussing the possibility of Apple rolling out a new beta program in the next few days with a cap of 100 beta partiticipants per app. In order to download a beta app, users will need to submit their iPhone's SN to the developer, who will then need to flag its eligibility in the store itself.

“Now we’re hearing from an app developer that Apple is finally going to start rolling out a new beta program in the next few days. Details are slim, but it seems like Apple is capping the total number of beta participants at 100 per app. In order to download a beta app, users will need to submit their iPhone’s serial number to the developer, who will then need to flag its eligibility in the store itself. All betas will still be distributed through the App Store - you won’t be able to download one on an external site.