Bruce M. Metzger on ἑπτά ("seven") in Revelation 5:6

  

5:6      τὰ [ἑπτά] {C}

The evidence for the presence of ἑπτά before πνεύματα (𝔓²⁴ א 046 1854 2053 2344 2432 it^gig syr^ph,  cop^sa,  arm Hippolytus al) is fairly evenly balanced against the evidence for its absence (A P^vid 1 1006 1611 it^ar vg eth Irenaeus^arm al). From the transcriptional point of view, through confusion with the two previous instances of ἑπτά in the preceding line, the word may have been accidentally omitted. On the other hand, copyists may have inserted the numeral in imitation of 1:4; 3:1; 4:5. In order to represent the ambiguities of external and internal considerations, the Committee decided to print the word, but to enclose it within square brackets, thus indicating doubt whether it belongs in the text. (Bruce M. Metzger, A Textual Commentary on the Greek New Testament: A Companion Volume to the United Bible Societies' Greek New Testament (Fourth Revised Edition) [2d ed.; Stuttgart, Germany: United Bible Societies, 1994], 666)