First, some vocabulary. While often used interchangeably, churning and app-o-rama are distinct:

Churning refers to applying for a credit card, earning the new account bonus, cancelling the card, and reapplying for the same card to earn another new account bonus. Not all issuers will permit this, and the time you must wait to cancel, or to reapply, varies by issuer and sometimes by specific card.

App-O-Rama (AOR) refers to submitting simultaneous applications for several credit cards, and earning the new account bonus on each. The reason for simultaneous application is to reduce the chance that an issuer will see that you have recently applied for credit elsewhere, or to include more than one card from the same issuer.