How Many HIFU Treatments is Needed to Achieve the Best Results?

The number of HIFU treatments required for the best results varies with each patient. Age and the conditions of the skin play a major.
hifu technology uses highly focused ultrasound energy to stimulate collagen regeneration in the layers under the skin. Because HIFU uses highly focused ultrasound to stimulate collagen under the skin to remodel, it takes a few months to see the full effect. 
The results of hifu therapy depends on the condition of the persons skin, age, medical condition and response of their own body's fibrillates to the stimulus. In general this means the younger the patient, the better the results, and menopause is an important issue since the lower estrogen levels contribute to thinning of the skin. Thin skin means less collagen as a "target" so less response. 
Generally, single treatment is enough to achieve good result with hifu. Most patients with early skin laxity only require one treatment to achieve noticeable results. However, some may benefit from more than one treatment depending on how much laxity they have and their body’s own biological response to the ultrasound and collagen-building process.
Most people have one hifu lifting treatment if their condition is mild to moderate and may have another treatment in a year to two. If some one is in the moderate to sever condition of sagging lose skin another treatment can be performed 6 months after the first treatment! 
It is safe to repeat it at yearly intervals but more frequent than that isn't going to add additional benefit. As collagen typically takes 6 to 12 months to develop, you need to wait this long between treatments to determine if repeating HIFU is necessary.