Update to Twitter lists: You can now make up to 1,000 lists (up from 20), and each list can include up to 5,000 accounts (up from 500).
— Twitter for News (@TwitterForNews) May 30, 2013
A much-needed update. You can now create up to 1,000 lists. And each list may include a maximum of 5,000 Tweeps.
Before the upgrade, users were limited to 20 lists and could only add up to 500 in each.