You're experiencing the beginning of voicing chords. Each and every chord will have 3, 4, and sometimes more notes. As in note names. Cmaj7 comprises C E G B. Put them in any order, play them, and there's a Cmaj7. You can have two of some if you like, and even miss out the G (that's another story for another day).
Once you get past triads (3 note chords, usually just major or minor), there's a problem with the voicing, as in which order they need to be in. This is where ears come to the fore. Listening to however many strings you play, and which notes are played on which strings, even for the same name chord, there'll be subtle differences in the sound. The term inversions crops up now - which note is actually where, importantly the lowest.
Which voicing and inversion you want will ( or should ) be decided by ear. With a Cmaj7 chord, there will be two notes which could be played a semitone apart. Technically o.k., but sound wise, not so good. With an open Cmaj7, there are several options available. Try out each, and decide which is best fit for you.
EDIT: to get to the lead question - a chord is usually three or more note played simultaneously. Some regard two as a chord, as in a 'power chord', or '5'. In simple terms, chords are usually what's called stacked thirds, where a root note has two other notes, a 3rd and another 3rd above. Usually maj3 and/or m3. Just about any three notes can and will constitute a chord, but some will sound good (CEG) others not so good (BbBC) Added to this is the naming of chords, which gets into a whole new ball game, and there's not room here for that one!