The Cm(maj7) is a minor chord with an additional major 7th (maj7). You normally get the maj7 by counting up the major(!!!) scale 7 tones from the root. Or just by going down a half-step (1 fret) from the root. Going up the minor scale always results in a minor 7th!
A Cm(maj7) has the notes C (1), Eb (m3), G (5) and B (maj7).
The Cm(maj7) chord is not a diatonic chord of the common major or minor scale. You can get it by stacking thirds of scale tones on the first scale degree of the C harmonic minor or C melodic minor scale.
Besides a moderen interpretation of jazz ballads, where replacing a m7 chord with a m(maj7) chord can provide an interesting flavor and tension, the m(maj7) chord can often be found in so called line clichés. That are chord progressions like Cm Cm(maj7) Cm7 Cm6, where just one single tone is moving up or down chromatically.
The use of a major 7th together with a minor chord is much more common in melodies and guitar solos than in the chord accompaniment.