Just took a SQ flight this evening and made an effort to check out the KrisShop magazine.

It was indicated in the magazine that the credit card limit is $600. Any amount greater than the card limit, the crew will need to make a call to the Credit Card Centre to get an approval code.

As such, I don't think there is a limit, but probably it will be a hassle if they cannot make a call out to get approval for whatever reasons.