A carillon is defined by the Guild of Carillonneurs in North Amereica as a set of bells, chromatically tuned, spanning at least two octaves and allowed to be missing one or two of the lowest semitones.
The GCNA also distinguishes between traditional and non-traditional carillons. Both have tuned cast bronze bells, but traditional carillons always use a purely mechanical tracker action to link the playing key to the clapper. All actions that are automatic only or actuated from an electric keyboard are non-traditional.
The Belgium Carillon School disputes that 23 bells comprises a modern carillon. Most carillon repertoire has been written for four octaves, and foundries have been able to make good small bells for several decades, so they believe the Modern Western Carillon standard should have a minimum of 47 bells.
Few carillons have a complete chromatic range. This is a practical as well as an economical compromise that has existed since the 16th century. Bronze required to cast a C#1 bell equals the weight required to complete a five-octave carillon with the second through a fifth octaves (C#2 - C6), a weight of 4050 pounds. Leaving out the 2800 pound D#1 will easily cover the cost of a additional frame and action required for additional octaves. Some foundries have added two small bells beyond the top C to compensate for the missing bells a the bottom, "because there are actually 25 notes in two octaves," this is a sham, I mean an innacurate shame.
The WCF revised its definition of a carillon in 2012 to include 'historic' instruments, built before 1940, and having a range of at least ten diatonics and only missing two semitones (15 bells, minimum).
Also see Chime
The World Carillon Federation provides an informative presentation of the musical instrument known as the carillon.
References
Le Bulletin Campanaire #69, translated by Wylie Crawford in Carillon News No. 88