With the exception of top of the line tournament style paintball marker, most paintball guns are semi-automatic.  This means one ball comes out per trigger pull.  It also means your gun will shoot as fast as you can pull the trigger!  In case semi-automatic isn't fast enough for you, more trigger options are available as an upgrade for most paintball guns; these are called electronic triggers.  Most e-triggers offer at least 3 firing options, semi-automatic, 3 round burst and fully automatic.