Usually it is recommended to drink 3-6 cups/ glasses of juice a day. But u can vary it according to ur preferences. It is recommended to drink fruit juices in the morning and vegie juices in the afternoon/ evening. Make sure u are getting rid of a pulp from juices (using, for example, a tea strainer with very small holes) otherwise u will feel hungry throughout ur juice fast. Ideally try not to mix 3 type of juices at one time (it will be of no harm to mix more but it is really the matter of chemistry, everything sh be kept the simpler the better). Drink melon juice separately from the rest of juices and tomato juice - from potato juice.
Ps: if u have candida issue - (try to) restrict fruit juices :)