The number of treatments required is completely dependent on the results that you want to achieve. If you want your tattoo fading, so that you can have a cover up, this will need fewer treatments than if you wanted the tattoo to be as invisible as possible.
It is recommended that you wait 4-6 weeks between each treatment to allow the area to fully heal. However, for medium to large sized tattoos, you can have other areas treated during this time.
Your specialist will be able to advise how many treatments are necessary to achieve your desired results during your initial consultation. This consultation also includes a patch test to ensure that you have no adverse reactions to the laser - and also to determine the required power needed to get the results.