In our long history working with spinal decompression we have treated hundreds of patients. Whether they were looking at spinal decompression as a surgical alternative, or it was recommended by us for certain patients that were not responding to traditional physical therapy and chiropractic manipulation, spinal decompression has been very effective and provided us with many satisfied patients, and more importantly, pain free patients.
Non surgical spinal decompression works using a method of decompression known as "non linear" traction. Through a slow and steady pulling and relaxing series of actions using precise amounts of weights, the patient feels a relaxing stretch in the lumbar spine, while within the spinal discs nutrition rushes in to the disc. Along with allowing more of the body's "nutritional" systems to help restore damaged tissue, pressure is also taken off of the disc, allowing the herniated material to receded back in to the disc. Through increasing the amount of oxygen/ blood/ nutritional flow to the disc, we can help speed up the body's natural healing process.
Decompression is not magic, so it requires a steady approach administered by a skilled physician. A single treatment might make you feel better immediately but it wont completely fix the underlying cause of pain, which is the damaged disc. A damaged disc requires time to heal. Only a full treatment 
protocol, consisting of a number of treatment treatments determined by your doctor can help reduce pain resulting from a herniated disc, bulging disc, ruptured disc, or symptoms of sciatica.
There is no set formula that applies to everyone - we use an individualized approach to treating our patients, which means we treat everyone as an individual and work closely with MD's and Physical therapists to work out a program that works for each individual. Therefore we have some variance in the number of treatments each patient receives. Usually we recommend between 15 to 20 treatment sessions.
After we start the decompression process, we can adjust the number of treatment sessions needed based on the body's response to treatment. Everyone is different, some people will require less treatment, some may have the number of their treatment sessions increased.
So the short answer is that the number of treatments depends on the individual and their diagnosis. This is a very good thing since a personally tailored plan will be the fastest possible protocol to get you back on track.