Normally, at least three transits are required to confirm a planet. Due to very high signal to noise ratio, only two transits were sufficient to validate Kepler-421b to be a real planet without additional confirmation methods.
Kepler-421b is slightly larger than Uranus although its mass is not known.
References
^Kipping, D. M.; Torres, G.; Buchhave, L. A.; Kenyon, S. J.; Henze, C.; Isaacson, H.; Kolbl, R.; Marcy, G. W.; Bryson, S. T.; Stassun, K.; Bastien, F. (2014), "Discovery of a Transiting Planet Near the Snow-Line", The Astrophysical Journal, 795 (1): 25, arXiv:1407.4807, Bibcode:2014ApJ...795...25K, doi:10.1088/0004-637X/795/1/25, S2CID119240746
Discovery of a Transiting Planet Near the Snow-Line, David M. Kipping, Guillermo Torres, Lars A. Buchhave, Scott J. Kenyon, Christopher E. Henze, Howard Isaacson, Rea Kolbl, Geoff W. Marcy, Stephen T. Bryson, Keivan G. Stassun, Fabienne A. Bastien, (Submitted on 17 Jul 2014)