Portable lighting

How many 500 watt portable halogen lights can you plug in to one 25 to 50 ft exention cord the link is the ones i am looking at

http://www.homedepot.com/h_d1/N-5yc...Id=-1&keyword=portable+lighting&storeId=10051

I'll assume it's a cord rated for 15A on a 15A circuit. Then 3 500 watt lights would be the max. 1500w / 125V = 12A

If it's one of those cords that use #16 wire it's only good for 13A, so only 2 of the lights can be used.