It is enough if you keep chanting the one Mantra that you feel attached to or that you have been initiated into by your Guru.  If, there happens to be a need to chant two or three different Mantras, allocate specific times or counts for each Mantra everyday and chant them accordingly. If your mind wavers to chant many different Mantras in the short span of time allotted to chant one, it will end up being counter-productive.