Interval class is used in two different ways. One, common in academic "set theory", defines it as the distance between two pitch classes, measured by the shortest distance. Thus C to G may be the interval of 7, but its interval class is 5. The largest interval class or "ic"--in 12edo--is the tritone (6). This may be criticized on two grounds: it's not a class in the vocabulary of mathematics, and it's less useful than the second definition.
The second definition, used for example by Scala, defines the interval class as the "generic interval" to which the specific intervals at a certain number of scale steps i apart belong. see also interval size measure, interval measure