Existing scars if close to an area for liposuction are preferentially used so as not to cause additional scaring. I also like to place them at the edge of a tattoo or in an existing scar such as a C-Section or appendectomy, or mole removal scar) so that they are hidden, or use the open area from an additional surgery I may be doing. For example, during a tummy tuck incisions are made to do liposuction in the area of redundant skin to be removed so that there are no extra scars.
Over the years the number of incision sites have decreased without compromise of results. There is no absolute number of incisions that need to be made - just enough to give the best result which will vary from plastic surgeon to plastic surgeon. Incisions sites for liposuction are typically very small and made in stealth areas - that is hidden as much as possible. For example inside the belly button, umbilicus, to approach the upper and lower abdomen, just at the pubic hair line for the lower tummy and pubis area, in the groin crease for the inside of the thighs (medial thighs), at the gluteal, buttock, crease for the lower back, in the breast crease for the fat beneath your arm (lateral chest wall), in the crease below your chin for liposuction of the neck.