Limit 1 per customer, don't think they really ever enforce this.
As promotions page says -
The $3.00 Suntory Boss Coffee Shake promotion is is available every Tuesday in March (7th, 14th, 21st and 28th) 2023 from 0h until 23.59h or during shop opening hours.
I had one with the blue can (double espresso) and it was pretty good, but next time ill ask for it in a smaller cup so the coffee taste is stronger.