Depends not only on the number of satellites that you are receiving information from but also which ones they are. The reason for that involves spherical trig, so it can be a little difficult unless you are fairly good at math.
The short answer is that the accuracy you read on the GPS is fairly accurate, and can vary from the above mentioned ± 2M to around ±15M.
One other thing that should be mentioned is that for maximum accuracy in re-finding a spot, mark your position and find your position in the same data system. GPS's will convert between different ones, but I've found that it does introduce another error.
Good luck! I carry mine everwhere!