1000 people is what they usually do for political polls. I was taught that a sample size of 1000 is sufficient for pretty much any purpose. At some point what becomes more significant is the differences between the poll-answering population and the general population. For eg if you call people at random on the phone, you're biased towards people who are likely to pick up the phone and talk to a pollster (eg retirees are home more often and not busy). Of course nobody is that naive nowadays, but that doesn't mean systemic error is completely eliminated.