There is no limit to non-refundable OBC which includes Shareholder, Amex Platinum benefit cash, NCL bonus cash, and some types of TA OBC.
They have a policy to limit refundable OBC to $1,000.
In my case I purchased $1,000 (to get Amex offer kickback). My TA tried to buy me additional OBC, on their credit card, and were denied by NCL as it would result in refundable OBC being over $1,000.