You can have any number of promotions linked to a session.
However, the user can only enter one coupon code during the registration process.
For example, let's assume you have 2 coupon codes EARLY5 ($5 off) and WELCOME10 (10% off) and both are linked to the same program. If the user has both these codes, they will only be able to enter one of them.
Users will not be able to stack coupon codes.