You need to be on the Nevada Salt Flats with nothing between you and the horizon in every direction to see all 12 satellites. 3 Are needed for a 2D fix, 4-5 for a 3D fix, and after that each satellite reduces the error in the fix and tracking by up to 25%.

What also matters is the number of correlators, you need one for each visible satellite as a minimum. The SiRF Star III had something crazy like 32000 correlators. This allows it to track at 20-30dB below aquisition, ie: indoors, in shipping containers, and in built up areas.