Two...one massive downhill track and one XC trail.
The XC is short and sweet with a degree of technicality which is good fun but easily doable by a 'newb'. I'd not stray onto the downhill track as you may find yourself in the back of a helicopter!
I lived in Cardiff a couple of years back and found myself at Afan Argoed most of my weekends. Much more trails on offer of a higher quality and greater length.
Cwn Carn is good and local for the Newport lot but I'd say the extra half hour in the car is worth it to get to Afan.