It depends upon what the contractor is talking about as the seal. Asphalt is put down in at least 2 lifts depending upon how thick you go. 4" to 6" would be fine for a driveway depending upon if any big vehicles drive in. The top lift uses a different oil and rock consistency for the final surface than the base lift. Both are done on a standard road job. Maybe the contractor is refering to this as a seal coat.
Some of the out of town/fly by night/show up at your door with a scam driveway contractors don't do a proper job and they take your money and leave you with a mess a couple years down the road when it breaks up. They only put down one thin lift with just one type of asphalt.
Also all asphalt needs to be maintained on a regular schedule. Around here it is preferred to seal coat the roads every 3 years although most places don't have the money to do it that often. There are different kinds of maintenance seal coats such as a chip/small rock seal or something else mixed with the oil. Some look like an oil surface and the chip seal is rock put on top of oil.
Contact your city/county engineer at the road department/public works for your locale and get their recommendations for a driveway. They should tell you for nothing.