I just discovered this bit of information, and thought it worthy of sharing. The Greek word μαθητής (mathetes, disciple, student) appears 261x in the Bible, but only in the NT (i.e., never in the LXX). Furthermore, it appears only in the Gospels and Acts. Here are the numbers.
Matthew: 72x, mostly speaking of the Twelve. Additionally, John the Baptist has disciples (11:2; 14:12), and the Pharisees do too (22:16), and Jesus speaks of a 'disciple' generally (10:24-25), and there do seem to be disciples of Jesus besides the Twelve (cf. 8:21). But it would be interesting to see if Matthew uses the word mathetes to describe followers of Jesus other than the Twelve after it is clear in Matthew's narrative that Jesus has specially commissioned the Twelve as his disciples (which apparently in Matthew happens in 10:2-4, where Matthew names the disciples). On a quick perusal, I did not find any instances.
Mark: 46x. Same issues as above. Interesting that the last appearance of mathetes is 16:7 (i.e., it does not appear in the longer ending of Mark). I'm not making an argument, just an observation. Of course, it doesn't appear in Mark's first chapter, either.
Luke: 37x. Weird that Luke, the longest Gospel, has fewer instances of mathetes than Mark, which Luke almost certainly knew (assuming Marcan priority, with the vast majority of scholars). Does this mean that Luke edited out the word mathetes from his Gospel? Does he have a word that he prefers to mathetes for naming the followers of Jesus? I'll try to think about those issues the next time I read through Luke. I'm not an NT scholar, so very probably if work has been done on this, it would have escaped my notice.
After typing the above, I looked through some of the verses in Luke, with some interesting initial results. Luke does use mathetes in reference to disciples of Jesus other than the Twelve--19:37: at the Triumphal Entry, "the whole crowd of the disciples" were rejoicing. Matthew (21:8-9) has only "crowd", and Mark (11:8) has "many" (πολλοί). Also, at 6:17, immediately after naming the Twelve, Luke tells us that a "large crowd of his disciples" were gathered to hear him.
Luke is also the Gospel that uses the word ἀπόστολος (apostolos) most frequently, though still not very often: Matthew uses it once (10:2), Mark once (6:30; but see the variant at 3:14), John once (13:16, but not in reference to the Twelve--just a general reference to 'one sent'), and Luke uses it six times (6:13; 9:10; 11:49; 17:5; 22:14; 24:10), at least five times in reference to the Twelve (maybe not 11:49). This of course does not count the many appearances of the word in Acts (28x), some of which do not refer to the Twelve (14:4, 14, in reference to Paul and Barnabas).
Back to mathetes:
John: 78x. I'm not interested right now in looking through these to see if they are always about the Twelve or are more general. A project for later.
Acts: 28x. I believe (from a quick perusal) that it is never used in reference to the Twelve in Acts. Instead, the Twelve are termed 'apostles', and mathetes is used in reference to general believers. See, for instance, ch. 6 (vv. 1, 2, 7), the first chapter where the term appears.
And that's it for the NT. Again, the word 'disciple' never appears in Paul's letters, or anyone else's letters. Christians are called 'saints' or 'believers' or 'brothers' (these terms are also used in Acts), but they are not called 'disciples' (or 'Christians', for the most part).