Is is possible to loose weight through massage?

Only through massage, you wouldn’t loose weight but connected with diet and physical exercise it helps the body to reduce water binding toxins that are mostly found in the grease layer underneath the skin. Massage increases the speed of metabolism so that the fat gets burned.

Is it possible to reduce cellulite through massage or to prevent it?

Since it is the water bound through toxins in the fat layer of the skin that makes cellulite, the answer would be the same like above. As better as getting rid of cellulite is prevention; less toxins, less fat, more fresh food and water and most important a skin with a well working blood circulation; dry brushing of the skin and cold showers in the morning are the best preventions.

How often should someone have a massage?

That certainly depends on the used massage techniques and the condition of the client. With the techniques I am using, 1-2 times per week is enough. Since I am using massage as a reaction therapy, I am waiting for the reaction of the body to go on.
5 times is usually enough to get a reaction for a change into the better. But for some people, especially those that know their body well and people practicing yoga and breathing techniques, one time for a certain problem can be enough.
If someone has come for more than 8-10 times and still doesn't feel any better, I am not the right person to treat him/her. My intention is not to treat the same people for the rest of their lives, so if I don't see any improvement at the first 4 times, I would kindly request them to look for someone different than me.
Of course that does not mean that someone who just enjoys massage should not come for the rest of his/her life as often as he/she wants.