It will depend on the heat (btu) rating, which dictates the fan size and thus the amperage. A 35,000 btu furnace typically draws 8-9 amps. A 40,000 btu model would likely be around 10-11 amps, while a 25,000 btu model would be in the 6-7 range.
If you know what model furnace you have, you can look up the specs online. The Ductek website sells most major makes and models and gives a pretty good description of each, including amperage.
It depends on the manufacturer and model. Atwood 8500 and Suburban SF series pull higher amps because the motors turn faster. 82 series and NT series pull a little less. DUO-Therm 659 series and older and Coleman furnaces are also slower motors and pull less amps. (Note: Hydroflame 82, DUO-Therm, and Coleman are no longer in production this is just info for those who may still have them.)