There are no limits!
For the first 25 Auto-Boosts, you will be charged at the end of the month.
However, following that, subsequent Auto-Boosts may be charged and deducted immediately (for example, your 26th Auto-Boost may be deducted immediately rather than at the end of the month).
Remember, you can manually Boost your plan at anytime through the Circles.Life app (1 GB and 2 GB) and you'll NEVER run out of data!