ShoeMoney started a thread at WebmasterWorld documenting that like with Google AdWords, MSN adCenter also has a keyword limit.
The keyword limit seems to be "somewhere around 40-50k." But you can set up about nine different accounts with that limit each. You can have up to three credit cards per account, and 3 accounts per credit card. Max that out.
Forum discussion at WebmasterWorld.
Update: See Microsoft adCenter Allows 100,000 Keywords Per Account posted on September 4, 2006.
