How much fat can be removed during liposuction?

No.In an outpatient setting (i.e. not a hospital OR), the limit is 5 liters (4 l in some states like Florida). When liposuction is done in combination with a second procedure, the limit is 1 liter.
These limits do not apply in a hospital setting. This is due to the fact that patients will be admitted and monitored overnight (thus allowing for better control of subsequent fluid shifts).
This will amount to approximately 11 lbs. Ultimately, it is important to remember that lipo is a contouring tool and not a means to weight loss. It is best reserved for addressing those areas which have been resistant to diet and exercise.
As always, discuss your concerns with a board certified plastic surgeon (ABPS).
Donovan Rosas MD
Board Certified by the American Board of Plastic Surgery
Member: American Society for Aesthetic Plastic Surgery, American Society of Plastic Surgeons
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Liposuction is a procedure for improving a person's silhouette. It is not meant for generalized weight loss. The amounts of fat removed does not correlate with weight loss. Most patients stay close to their normal weight after a typical liposuction procedure. Small amounts (on the order of 5 lbs, not 20) of weight loss can occur with larger volumes removed and a patient doing their part after with diet and eventually exercise. 

Yes, it is possible but would be unsafe unless in very experienced hands, and is not indicated for weight loss. For body contouring the total amount could be removed in two staged procedures, for example, thighs, calves and ankles, 4-6 liters each time.  This should not be done for weight loss, but if done by someone who understands fluid balance, knows how to monitor the patient closely, or even admits the patient to a facility for overnight monitoring, it can and has been done safely.  Some states, like Florida, forbid it, because of safety concerns and deaths associated with outpatient liposuction. It is only for experts. Dr. Commons published his results 20 years ago and had a dedicated, experienced anesthesiologist and team.Plast Reconstr Surg. 2001 Nov;108(6):1753-63; discussion 1764-7.

Large-volume liposuction: a review of 631 consecutive cases over 12 years.

Commons GW1, Halperin B, Chang CC.Since the advent of epinephrine-containing wetting solutions and sophisticated fluid management techniques, increasingly larger and larger volumes of liposuction aspirations have been reported. Unfortunately, with these larger volumes of liposuction being routinely performed, greater rates of complications have also been reported, with the worst of these resulting in deaths. In a response to the increasing concerns over the safety of large-volume liposuction, a critical review of the senior author's own series has been performed to evaluate risks and benefits and to recommend guidelines for safe and effective large-volume liposuction. A retrospective chart review was performed on 631 consecutive patients who underwent liposuction procedures of at least 3000 cc total aspirate. All procedures were performed by the same senior surgeon between January of 1986 and March of 1998. Before September of 1996, traditional liposuction techniques were used. After September of 1996, ultrasound-assisted liposuction was performed. The superwet technique of fluid management was employed for all procedures performed after 1991. The particulars of the surgical and anesthetic techniques used are reviewed in the article. Data collection included preoperative patient demographics, preoperative and postoperative weights and measurements, and preoperative and postoperative photographs. Total aspirate volumes, fluid intakes, and fluid outputs were measured, and all complications were tallied. Average follow-up was 1 year. Results showed the majority of patients to be women, aged 17 to 74 years old. Of the preoperative weights, 98.7 percent were within 50 pounds of ideal chart weight. Total aspirate volumes ranged from 3 to 17 liters, with 94.5 percent of these under 10 liters. Fluid balance measurements showed an average of 120 cc/kg positive fluid balance at the end of the procedure, with none of these patients experiencing any significant fluid balance abnormalities. Cosmetic results were good, with a 2- to 6-inch drop from preoperative measurements, depending on the area treated. Ten percent of patients experienced minor skin contour irregularities, with most of these patients not requiring any additional surgical procedures. One year after surgery, 80 percent of patients maintained stable postoperative weights. No serious complications were experienced in this series. The majority of the complications consisted of minor skin injuries and burns, allergic reactions to garments, and postoperative seromas. The more serious complications included four patients who developed mild pulmonary edema and one patient who developed pneumonia postoperatively. These patients were treated appropriately and went on to have uneventful recoveries. The results show that large-volume liposuction can be a safe and effective procedure when patients are carefully selected and when anesthetic and surgical techniques are properly performed. Meticulous fluid balance calculations are necessary to avoid volume abnormalities, and experience is mandatory when performing the largest aspirations. Cosmetic benefits are excellent, and overall complication rates are low.