How many treatments of microdermabrasion for hyperpigmentations and moderate acne scars? And how long?

Dermabrasion is an adequate treatment for some types of scarring/dyspigmenation but deeper ice pick and rolling scars may need deeper treatments.  Hydroquinone  can be used to prevent untoward side effects (e.g. dyspigmentation) that occurs after the procedure.  Not all dermatologist prescribe this prior to or post treatment. A better option may be lasers such as carbon dioxide lasers like Deep Fx and Active Fx.  These lasers allow for adequate treatment depth that the dermabrasion may not. They reduce the depth of the scar, discoloration, and tighten the collagen.  Overall, there is an improvement in color, texture, and depth of scarring.  Prior to starting a treatment regimen I would consult a Board Certified dermatologist or plastic surgeon who has expertise in lasers and scar treatment. 

Microdermabrasion is a very superficial mechanical exfoliation of the skin.  It is an excellent treatment for smoothing and softening the skin and is great to do for general anti-aging effects.  However, it is unlikely that you will experience significant improvement in acne scars or in hyperpigmentation with this superficial of a treatment.  I would recommend a series of microneedling treatments combined with PRP (protein rich plasma).  This is a more aggressive treatment that can significantly improve both acne scars and hyperpigmentation.  You should arrange a consultation with a physician who specializes in aesthetic medicine.

Microderm for someone with moderate scars and hyper pigmentation should do a series of at least 3 but I would recommend you do about 6 to see a noticeable difference. Since Microderm is not a super deep exfoliation of the skin, it is imperative that you do many and closely spaced together (every 2-3 weeks) for best results. 
Not all doctors prescribe hydroquinone with this treatment. But, hydroquinone will very effectively help definitely help diminish the look of dark spots and inconsistent skin tone. Just make sure you use sunscreen anytime you're using hydroquinone.

Micro-dermabrasion alone is not an effective treatment for the conditions you are asking about. For treatment of hyperpigmentation as well as acne scars, we are getting excellent results combining micro-needling with micro-dermabrasion and application of human growth-factor serums.Combining micro-needling with micro-dermabrasion causes even greater penetration of topical serums and medications which will further enhance the effect of human growth-factor serums in repairing acne scars and lighten hyperpigmentation. Up to 80% of product can penetrate the skin compared to 7 to 10% with normal topical application.
In my office, we use the ProCell Therapies system which combines motorized micro-needling with micro-dermabrasion and also uses concentrated human growth factor serums made from human stem cells. Our patients also use growth factor serum at home after their office treatment.