The Cmaj7(13) or just Cmaj13 is a Cmaj7 chord with an additional major 6th A. Because the 6th is extending the 7th chord, rather than replacing the 7th, it is written as 13 in the chord symbol.
A Cmaj7(13) has the notes C (1), E (3), G (5), B (maj7) and A (13). The 5th can be omitted respectively abandoned for the 13.
On the guitar you can start with a Cmaj7 chord shape, search for the 5th G and move it up 2 frets (a whole-step) to the 6th/13 A. If your chord shape has two 5ths, I'd recommend to take the higher one for that.
You will rarely see a maj7(13) chord written on a music sheet. While the maj9 chord sounds pretty sweet and common, the 13 adds a quite special flavour that is not always desired. On the other side, the sound of a maj13 has kind of a high recognition factor, that can make song or part of it very unique.