You are asking a very simple and yet complex question. It depends what is being injected - is it cosmetic and inject everything that visible or are there 'feeder veins' that also have to be injected in a serial fashion - feeder veins first and return for spider and reticular viens. I would leave this to the discretion of the treating physician and please choose one who is board certified from the Am College of Venous and Lymphatic Medicine - prveiously called American Board of Phleboogy.
Sclerotherapy involves multiple injections using very small needles. Many veins interconnect so that usually not every vein needs to be injected. The principle of sclerotherapy treatment is to inject from the bigger to the smaller veins. Often times the bigger veins feed into the smaller ones. The sclerotherapy solution is injected either as a liquid or as a foam depending the the veins and the doctor experience. These treatments are very well tolerated.
The solution is usually injected into a larger vein that feeds a network of smaller veins. The injector can see the color change as the fluid flows through the veins and they turn white. This is the best way to know what areas have been treated. Color usually returns quickly but the injury to the vein has been done. It is not necessary to inject along the entire length of a vein.