Who will be the first and how many Cardinals will resign in the 21st century?

Resigning Cardinals

15th century
Otón de Moncada i de Luna (pseudocardinal, 1449)
Wincenty Kot z Dębna (pseudocardinal, 1449)
Bartolomeo Vitelleschi (pseudocardinal, 1449)
Ardicino Della Porta, iuniore (1489)
Cesare Borgia (1493)

16th century
Ferdinando de' Medici (1563)
Albrecht von Austria (1577)
Fernando de Toledo Oropesa (1578)

17th century
Maurizio di Savoia (1607)
Ferdinando Gonzaga (1607)
Camillo Francesco Maria Pamphilj (1644)
Jan Kazimierz Waza, S.J. (1646)
Francesco Maria de' Medici (1686)
Rinaldo d'Este (1686)

18th century
Gabriele Filippucci (1706)
Luis Antonio Jaime de Borbón y Farnesio (1735)
Vincenzo Maria Altieri (1777)
Étienne-Charles de Loménie de Brienne (1788)
Tommaso Antici (1789)

19th century
Marino Carafa di Belvedere (1801)
Carlo Odescalchi (1823)

20th century
Louis Billot, S.J. (1911)

The Cardinals of the Holy Roman Church - Dimissionary cardinals Cardinal O'Brien may be adding to the list shortly.