The
License FAQ has good info. To sum up the main point "each" and every computer/laptop/iPad needs its own
UNIQUE license number. Reusing a license will cause issues.
HL comes with two licenses as part of the 30 bucks. This allows you to use one on your main computer and one on your iPad. They will share the same content so you only have to buy a package (ie Ultimate Magic) one time and both devices will have access.
Also if you want more licenses they are 10 bucks and you can have up to 5 of them now.