20 credit maximum
As an Airtran elite, one of the best benefits was the ability to transfer my credits to other accounts without fee. In this way, if my wife has two credits stranded in her account with no likelihood of earning the other 6 necessary for a ticket, I could transfer 6 of mine and she could earn a ticket (and use the ticket for me!).
However, I have just come up against a previously unknown rule: the maximum number of credits you can transfer each year is 20.