How much can I smoke and still be safe? How many packs of cigarettes do you need to smoke or how many cigarettes a week before you should get nervous about cancer?

Abstinence is best. There is no magic number. One of the best things you can do for your health including decreasing your risk for cancer and other health problems is to stop smoking immediately and not be around second hand smoke.
Any . Any amount of smoking, including even passive smoking from the inhalation of environmental tobacco smoke has been proven to cause an increased risk of developing cancer. There is therefore no "safe" quantity of smoking cigarettes when it comes to lung (and many other types) of cancer risk. The same is true for smoking in relation to cardiovascular risk --heart attacks, stroke and peripheral vascular disease. Smokers are approximately 20-30 times more likely than non-smokers to develop lung cancer over their lifetime. It is also well established that the greater the number of cigarettes that you smoke in your lifetime, the higher the risk of developing lung cancer. As a result, heavy smokers are even more at risk of developing lung cancer than occasional smokers. Additional factors that are thought to increase the likelihood of developing lung cancer in smokers includes the age at the onset of smoking, degree of inhalation, tar and nicotine content of the cigarettes and finally, the use of unfiltered cigarettes. Bottom line --all smokers should be nervous about developing cancer.