There are limits. Currently, the program is only good through the end of next year, and if you spend more than $25,000 you stop getting bonuses.
From the Disney Vacation Account site:
Disney Vacation Account Bonus Program
What is the Bonus Program?
You'll be eligible for a $20 Disney Gift Card for every $1000 you spend on qualified vacation purchases with your Disney Vacation Account prior to December 31, 2017 (date subject to extension), up to a total maximum of $500 in Disney Gift Card(s) per household. Your Disney Vacation Account must be active for a minimum of 120 days at the time of spending in order to receive this bonus. All accounts registered to the same address are considered part of the same household.
Qualified vacation purchases include vacation packages for the Walt Disney World Resort or the
Disneyland Resort, theme park tickets, room stays, cruise fare, and vacations by
Adventures By Disney. Purchases of Disney Gift Cards are not qualified vacation purchases for purposes of this Bonus Program.
Request your Disney Gift Card(s) 4 to 6 weeks in advance of your vacation date to allow time for delivery to your mailing address.